4.1: Frog Embryology - Biology

The Egg

Figure 14.2.1 Frog Egg

The frog egg is a huge cell; its volume is over 1.6 million times larger than a normal frog cell. During embryonic development, the egg will be converted into a tadpole containing millions of cells but containing the same amount of organic matter.

  • The upper hemisphere of the egg — the animal pole — is dark.
  • The lower hemisphere — the vegetal pole — is light.
  • When deposited in the water and ready for fertilization, the haploid egg is at metaphase of meiosis II.


Figure 14.2.2 Frog Zygote

Entrance of the sperm initiates a sequence of events:

  • Meiosis II is completed.
  • The cytoplasm of the egg rotates about 30 degrees relative to the poles.
  • In some amphibians (including Xenopus), this is revealed by the appearance of a light-colored band, the gray crescent.
  • The gray crescent forms opposite the point where the sperm entered.
  • It foretells the future pattern of the animal: its dorsal (D) and ventral (V) surfaces; its anterior (A) and posterior (P); its left and right sides.
  • The haploid sperm and egg nuclei fuse to form the diploid zygote nucleus.


The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells. The first cleavage occurs shortly after the zygote nucleus forms. A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. This divides the egg into two halves forming the 2-cell stage. The second cleavage forms the 4-cell stage. The cleavage furrow again runs through the poles but at right angles to the first furrow. The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal pole. It produces the 8-cell stage.

Figure 14.2.3 Various stages of cleavage in a frog zygote

The next few cleavages also proceed in synchrony, producing a 16-cell and then a 32-cell embryo. However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus become smaller and more numerous. By the next day, continued cleavage has produced a hollow ball of thousands of cells called the blastula. A fluid-filled cavity, the blastocoel, forms within it.

Figure 14.2.4 Frog Bastula

During this entire process there has been no growth of the embryo. In fact, because the cells of the blastula are so small, the blastula looks just like the original egg to the unaided eye. Not until the blastula contains some 4,000 cells is there any transcription of zygote genes. All of the activities up to now have been run by gene products (mRNA and proteins) deposited by the mother when she formed the egg.


The start of gastrulation is marked by the pushing inward ("invagination") of cells in the region of the embryo once occupied by the middle of the gray crescent.

Figure 14.2.5 Frog gastrula

This produces an opening (the blastopore) that will be the future anus. a cluster of cells that develops into the Spemann organizer (named after one of the German embryologists who discovered its remarkable inductive properties).

As gastrulation continues, three distinct "germ layers" are formed:

  • ectoderm
  • mesoderm
  • endoderm

Each of these will have special roles to play in building the complete animal. Some are listed in the table.

Germ-layer origin of various body tissues
skinnotochordinner lining of gut, liver, pancreas
brainmusclesinner lining of lungs
spinal cordbloodinner lining of bladder
all other neuronsbonethyroid and parathyroid glands
sense receptorssex organsthymus

Figure 14.2.6 Frog neural folds

The Spemann organizer (mostly mesoderm) will develop into the notochord, which is the precursor of the backbone and induce the ectoderm lying above it to begin to form neural tissue instead of skin. This ectoderm grows up into two longitudinal folds, forming the neural folds stage. In time the lips of the folds fuse to form the neural tube. The neural tube eventually develops into the brain and spinal cord.


Although the various layers of cells in the frog gastrula have definite and different fates in store for them, these are not readily apparent in their structure. Only by probing for different patterns of gene expression (e.g., looking for tissue-specific proteins) can their differences be detected. In due course, however, the cells of the embryo take on the specialized structures and functions that they have in the tadpole, forming neurons, blood cells, muscle cells, epithelial cells, etc., etc.


At the time the tadpole hatches, it is a fully-formed organism. However, it has no more organic matter in it than the original frog egg had. Once able to feed, however, the tadpole can grow. It gains additional molecules with which it can increase the number of cells that make up its various tissues.

The extraordinary biology and development of marsupial frogs (Hemiphractidae) in comparison with fish, mammals, birds, amphibians and other animals

The study of oogenesis and early development of frogs belonging to the family Hemiphractidae provide important comparison to the aquatic development of other frogs, such as Xenopus laevis, because reproduction on land characterizes the Hemiphractidae. In this review, the multinucleated oogenesis of the marsupial frog Flectonotus pygmaeus (Hemiphractidae) is analyzed and interpreted. In addition, the adaptations associated with the incubation of embryos in the pouch of the female marsupial frog Gastrotheca riobambae (Hemiphractidae) and the embryonic development of this frog are summarized. Moreover, G. riobambae gastrulation is compared with the gastrulation modes of Engystomops randi and Engystomops coloradorum (Leptodactylidae) Ceratophrys stolzmanni (Ceratophryidae) Hyalinobatrachium fleischmanni and Espadarana callistomma (Centrolenidae) Ameerega bilinguis, Dendrobates auratus, Epipedobates anthonyi, Epipedobates machalilla, Epipedobates tricolor, and Hyloxalus vertebralis (Dendrobatidae) Eleutherodactylus coqui (Terrarana: Eleutherodactylidae), and X. laevis (Pipidae). The comparison indicated two modes of frog gastrulation. In X. laevis and in frogs with aquatic reproduction, convergent extension begins during gastrulation. In contrast, convergent extension occurs in the post-gastrula of frogs with terrestrial reproduction. These two modes of gastrulation resemble the transitions toward meroblastic cleavage found in ray-finned fishes (Actinopterygii). In spite of this difference, the genes that guide early development seem to be highly conserved in frogs. I conclude that the shift of convergent extension to the post-gastrula accompanied the diversification of frog egg size and terrestrial reproductive modes.

Biology Questions: Frog Embryology

This material may consist of step-by-step explanations on how to solve a problem or examples of proper writing, including the use of citations, references, bibliographies, and formatting. This material is made available for the sole purpose of studying and learning - misuse is strictly forbidden.

12. Which ball of cells/embryonic stage is impermeable to water? Which is permeable to water? What does it mean to be impermeable or permeable? Cells till Morula stage are impermeable to water. After 128 cell stage of Morula, the cells become permeable to water. By permeability it means that the cells would allow water to enter the morula.
13. When NaCl moves inside the morula, it changes the water concentration there (Figure 6). Because the water concentration has decreased additional water will follow and we find a shift to the next stage of development. What is the term used to describe such passive movement of water? Diffusion/ Osmosis
What term is correct when describing the morula compared to an environment of pure water: isotonic, hypertonic or hypotonic? Hypertonic.

This is only a preview of the solution. Please use the purchase button to see the entire solution

Primary neurulation

The events of primary neurulation in the chick and the frog are illustrated in Figures 12.3 and 12.4, respectively. During primary neurulation, the original ectoderm is divided into three sets of cells: (1) the internally positioned neural tube, which will form the brain and spinal cord, (2) the externally positioned epidermis of the skin, and (3) the neural crest cells. The neural crest cells form in the region that connects the neural tube and epidermis, but then migrate elsewhere they will generate the peripheral neurons and glia, the pigment cells of the skin, and several other cell types.

Figure 12.3

Primary neurulation: neural tube formation in the chick embryo. (A, 1) Cells of the neural plate can be distinguished as elongated cells in the dorsal region of the ectoderm. Folding begins as the medial neural hinge point (MHP) cells anchor to notochord (more. )

Figure 12.4

Three views of neurulation in an amphibian embryo, showing early (left), middle (center), and late (right) neurulae in each case. (A) Looking down on the dorsal surface of the whole embryo. (B) Sagit-tal section through the medial plane of the embryo. (more. )


Chick Neurulation. By 33 hours of incubation, neurulation in the chick embryo is well underway. Both whole mounts and a complete set of serial cross sections through a 33-hour chick embryo are included in this segment so that you can see this amazing event. The serial sections can be displayed either as a continuum in movie format or individually, along with labels and color-coding that designates germ layers. [Click on Chick-Mid]

The process of primary neurulation appears to be similar in amphibians, reptiles, birds, and mammals (Gallera 1971). Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds, while a U-shaped neural groove appears in the center of the plate, dividing the future right and left sides of the embryo (see Figures 12.2C and 12.3). The neural folds migrate toward the midline of the embryo, eventually fusing to form the neural tube beneath the overlying ectoderm. The cells at the dorsalmost portion of the neural tube become the neural crest cells.

Neurulation occurs in somewhat different ways in different regions of the body. The head, trunk, and tail each form their region of the neural tube in ways that reflect the inductive relationship of the pharyngeal endoderm, prechordal plate, and notochord to its overlying ectoderm (Chapters 10 and 11). The head and trunk regions both undergo variants of primary neurulation, and this process can be divided into four distinct but spatially and temporally overlapping stages: (1) formation of the neural plate (2) shaping of the neural plate (3) bending of the neural plate to form the neural groove and (4) closure of the neural groove to form the neural tube (Smith and Schoenwolf 1997 see Figure 12.2).

Formation and shaping of the neural plate

The process of neurulation begins when the underlying dorsal mesoderm (and pharyngeal endoderm in the head region) signals the ectodermal cells above it to elongate into columnar neural plate cells (Smith and Schoenwolf 1989 Keller et al. 1992 ). Their elongated shape distinguishes the cells of the prospective neural plate from the flatter pre-epidermal cells surrounding them. As much as 50% of the ectoderm is included in the neural plate. The neural plate is shaped by the intrinsic movements of the epidermal and neural plate regions. The neural plate lengthens along the anterior-posterior axis, narrowing itself so that subsequent bending will form a tube (instead of a spherical capsule).

In both amphibians and amniotes, the neural plate lengthens and narrows by convergent extension, intercalating several layers of cells into a few layers. In addition, the cell divisions of the neural plate cells are preferentially in the rostral-caudal (beak-tail anterior-posterior) direction (Jacobson and Sater 1988 Schoenwolf and Alvarez 1989 Sausedo et al. 1997 see Figures 12.2 and 12.3). These events will occur even if the tissues involved are isolated. If the neural plate is isolated, its cells converge and extend to make a thinner plate, but fail to roll up into a neural tube. However, if the 𠇋order region” containing both presumptive epidermis and neural plate tissue is isolated, it will form small neural folds in culture (Jacobson and Moury 1995 Moury and Schoenwolf 1995).


12.1 Formation of the floor plate cells. One of the major controversies in developmental neurobiology concerns the origin of the cells that form the ventral floor of the neural tube. It is possible that these cells are derived directly from the notochord and do not arise from the surface ectoderm.

Bending of the neural plate

The bending of the neural plate involves the formation of hinge regions where the neural tube contacts surrounding tissues. In these regions, the presumptive epidermal cells adhere to the lateral edges of the neural plate and move them toward the midline (see Figure 12.3B). In birds and mammals, the cells at the midline of the neural plate are called the medial hinge point (MHP) cells. They are derived from the portion of the neural plate just anterior to Hensen's node and from the anterior midline of Hensen's node (Schoenwolf 1991a,b Catala et al. 1996). The MHP cells become anchored to the notochord beneath them and form a hinge, which forms a furrow at the dorsal midline. The notochord induces the MHP cells to decrease their height and to become wedge-shaped (van Straaten et al. 1988 Smith and Schoenwolf 1989). The cells lateral to the MHP do not undergo such a change (Figures 12.3B,C). Shortly thereafter, two other hinge regions form furrows near the connection of the neural plate with the remainder of the ectoderm. These regions are called the dorsolateral hinge points (DLHPs), and they are anchored to the surface ectoderm of the neural folds. These cells, too, increase their height and become wedge-shaped.

Cell wedging is intimately linked to changes in cell shape. In the DLHPs, microtubules and microfilaments are both involved in these changes. Colchicine, an inhibitor of microtubule polymerization, inhibits the elongation of these cells, while cytochalasin B, an inhibitor of microfilament formation, prevents the apical constriction of these cells, thereby inhibiting wedge formation (Burnside 1973 Karfunkel 1972 Nagele and Lee 1987). After the initial furrowing of the neural plate, the plate bends around these hinge regions. Each hinge acts as a pivot that directs the rotation of the cells around it (Smith and Schoenwolf 1991).

Meanwhile, extrinsic forces are also at work. The surface ectoderm of the chick embryo pushes toward the midline of the embryo, providing another motive force for the bending of the neural plate (see Figure 12.3C Alvarez and Schoenwolf 1992). This movement of the presumptive epidermis and the anchoring of the neural plate to the underlying mesoderm may also be important for ensuring that the neural tube invaginates into the embryo and not outward. If small pieces of neural plate are isolated from the rest of the embryo (including the mesoderm), they tend to roll inside out (Schoenwolf 1991a). The pushing of the presumptive epidermis toward the center and the furrowing of the neural tube creates the neural folds.

Closure of the neural tube

The neural tube closes as the paired neural folds are brought together at the dorsal midline. The folds adhere to each other, and the cells from the two folds merge. In some species, the cells at this junction form the neural crest cells. In birds, the neural crest cells do not migrate from the dorsal region until after the neural tube has been closed at that site. In mammals, however, the cranial neural crest cells (which form facial and neck structures) migrate while the neural folds are elevating (i.e., prior to neural tube closure), whereas in the spinal cord region, the crest cells wait until closure has occurred (Nichols 1981 Erickson and Weston 1983).

The closure of the neural tube does not occur simultaneously throughout the ectoderm. This is best seen in those vertebrates (such as birds and mammals) whose body axis is elongated prior to neurulation. Figure 12.5 depicts neurulation in a 24-hour chick embryo. Neurulation in the cephalic (head) region is well advanced, while the caudal (tail) region of the embryo is still undergoing gastrulation. Regionalization of the neural tube also occurs as a result of changes in the shape of the tube. In the cephalic end (where the brain will form), the wall of the tube is broad and thick. Here, a series of swellings and constrictions define the various brain compartments. Caudal to the head region, however, the neural tube remains a simple tube that tapers off toward the tail. The two open ends of the neural tube are called the anterior neuropore and the posterior neuropore.

Figure 12.5

Stereogram of a 24-hour chick embryo. Cephalic portions are finishing neurulation while the caudal portions are still undergoing gastrulation. (From Patten 1971 after Huettner 1949.)

Unlike neurulation in chicks (in which neural tube closure is initiated at the level of the future midbrain and “zips up” in both directions), neural tube closure in mammals is initiated at several places along the anterior-posterior axis (Golden and Chernoff 1993 Van Allen et al. 1993). Different neural tube defects are caused when various parts of the neural tube fail to close (Figure 12.6). Failure to close the human posterior neural tube regions at day 27 (or the subsequent rupture of the posterior neuropore shortly thereafter) results in a condition called spina bifida, the severity of which depends on how much of the spinal cord remains exposed. Failure to close the anterior neural tube regions results in a lethal condition, anencephaly. Here, the forebrain remains in contact with the amniotic fluid and subsequently degenerates. Fetal forebrain development ceases, and the vault of the skull fails to form. The failure of the entire neural tube to close over the entire body axis is called craniorachischisis. Collectively, neural tube defects are not rare in humans, as they are seen in about 1 in every 500 live births. Neural tube closure defects can often be detected during pregnancy by various physical and chemical tests.

Figure 12.6

Neurulation in the human embryo. (A) Dorsal and transverse sections of a 22-day human embryo initiating neurulation. Both anterior and posterior neuropores are open to the amniotic fluid. (B) Dorsal view of a neurulating human embryo a day later. The (more. )

Human neural tube closure requires a complex interplay between genetic and environmental factors. Certain genes, such as Pax3, sonic hedgehog, and openbrain, are essential for the formation of the mammalian neural tube, but dietary factors, such as cholesterol and folic acid, also appear to be critical. It has been estimated that 50% of human neural tube defects could be prevented by a pregnant woman's taking supplemental folic acid (vitamin B12), and the U.S. Public Health Service recommends that all women of childbearing age take 0.4 mg of folate daily to reduce the risk of neural tube defects during pregnancy (Milunsky et al. 1989 Czeizel and Dudas 1992 Centers for Disease Control 1992).

The neural tube eventually forms a closed cylinder that separates from the surface ectoderm. This separation is thought to be mediated by the expression of different cell adhesion molecules. Although the cells that will become the neural tube originally express E-cadherin, they stop producing this protein as the neural tube forms, and instead synthesize N-cadherin and N-CAM (Figure 12.7). As a result, the surface ectoderm and neural tube tissues no longer adhere to each other. If the surface ectoderm is experimentally made to express N-cadherin (by injecting N-cadherin mRNA into one cell of a 2-cell Xenopus embryo), the separation of the neural tube from the presumptive epidermis is dramatically impeded (Detrick et al. 1990 Fujimori et al. 1990).

Figure 12.7

Expression of N-cadherin and E-cadherin adhesion proteins during neurulation in Xenopus. (A) Normal development. In the neural plate stage, N-cadherin is seen in the neural plate, while E-cadherin is seen on the presumptive epidermis. Eventually, the (more. )


12.2 Neural tube closure. The closing of the neural tube is a complex event that can be influenced by both genes and environment. The interactions between genetic and environmental factors are now being untangled.


Preformationism and epigenesis Edit

As recently as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that simply becomes larger during development.

The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. Much early embryology came from the work of the Italian anatomists Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, and Mauro Ruscóni. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

'CLEVAGE' Cleavage is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the egg is fertilized by the sperm. The ways in which the cells divide is specific to certain types of animals and may have many forms.

Holoblastic Edit

Holoblastic cleavage is the complete division of cells. Holoblastic cleavage can be radial (see: Radial cleavage), spiral (see: Spiral cleavage), bilateral (see: Bilateral cleavage), or rotational (see: Rotational cleavage). In holoblastic cleavage the entire egg will divide and become the embryo, whereas in meroblastic cleavage some cells will become the embryo and others will be the yolk sac.

Meroblastic Edit

Meroblastic cleavage is the incomplete division of cells. The division furrow does not protrude into the yolky region as those cells impede membrane formation and this causes the incomplete separation of cells. Meroblastic cleavage can bilateral (see: Bilateral cleavage), discoidal (see: Discoidal cleavage), or centrolecithal (see: Centrolecithal).

Basal phyla Edit

Animals that belong to the basal phyla have holoblastic radial cleavage which results in radial symmetry (see: Symmetry in biology). During cleavage there is a central axis that all divisions rotate about. The basal phyla also has only one to two embryonic cell layers, compared to the three in bilateral animals (endoderm, mesoderm, and ectoderm).

Bilaterians Edit

In bilateral animals cleavage can be either holoblastic or meroblastic depending on the species. During gastrulation the blastula develops in one of two ways that divide the whole animal kingdom into two-halves (see: Embryological origins of the mouth and anus). If in the blastula the first pore, or blastopore, becomes the mouth of the animal, it is a protostome if the blastopore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula. Soon after the gastrula is formed, three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop.

Germ layers Edit

  • The innermost layer, or endoderm, give rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
  • The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system.
  • The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.

Drosophila melanogaster (fruit fly) Edit

Drosophila have been used as a developmental model for many years. The studies that have been conducted have discovered many useful aspects of development that not only apply to fruit flies but other species as well.

Outlined below is the process that leads to cell and tissue differentiation.

    help to define the anterior-posterior axis using Bicoid (gene) and Nanos (gene). establish 3 broad segments of the embryo. define 7 segments of the embryo within the confines of the second broad segment that was defined by the gap genes. define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves using a gradient of Hedgehog and Wnt. use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type.

Humans Edit

Humans are bilateral animals that have holoblastic rotational cleavage. Humans are also deuterostomes. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

Evolutionary embryology is the expansion of comparative embryology by the ideas of Charles Darwin. Similarly to Karl Ernst von Baer's principles that explained why many species often appear similar to one another in early developmental stages, Darwin argued that the relationship between groups can be determined based upon common embryonic and larval structures.

Von Baer's principles Edit

  1. The general features appear earlier in development than do the specialized features.
  2. More specialized characters develop from the more general ones.
  3. The embryo of a given species never resembles the adult form of a lower one.
  4. The embryo of a given species does resemble the embryonic form of a lower one. [1]

Using Darwin's theory evolutionary embryologists have since been able to distinguish between homologous and analogous structures between varying species. Homologous structures are those that the similarities between them are derived from a common ancestor, such as the human arm and bat wings. Analogous structures are those that appear to be similar but have no common ancestral derivation. [1]

Until the birth of modern embryology through observation of the mammalian ovum by Karl Ernst von Baer in 1827, there was no clear scientific understanding of embryology. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available. Karl Ernst von Baer along with Heinz Christian Pander, also proposed the germ layer theory of development which helped to explain how the embryo developed in progressive steps. Part of this explanation explored why embryos in many species often appear similar to one another in early developmental stages using his four principles.

Modern embryology research Edit

Embryology is central to evolutionary developmental biology ("evo-devo"), which studies the genetic control of the development process (e.g. morphogens), its link to cell signalling, its roles in certain diseases and mutations, and its links to stem cell research. Embryology is the key to Gestational Surrogacy, which is when the sperm of the intended father and egg of intended mother are fused in a lab forming an embryo. This embryo is then put into the surrogate who carries the child to term.

Medical embryology is used widely to detect abnormalities before birth. 2-5% of babies are born with an observable abnormality and medical embryology explores the different ways and stages that these abnormalities appear in. [1] Genetically derived abnormalities are referred to as malformations. When there are multiple malformations, this is considered a syndrome. When abnormalities appear due to outside contributors, these are disruptions. The outside contributors causing disruptions are known as teratogens. Common teratogens are alcohol, retinoic acid, [2] ionizing radiation or hyperthermic stress.

Many principles of embryology apply to invertebrates as well as to vertebrates. Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage. For decades, a number of so-called normal staging tables were produced for the embryology of particular species, mainly focussing on external developmental characters. As variation in developmental progress makes comparison among species difficult, a character-based Standard Event System was developed, which documents these differences and allows for phylogenetic comparisons among species. [3]

After the 1950s, with the DNA helical structure being unraveled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

Human embryo at six weeks gestational age

Histological film 10-day mouse embryo

As of today, human embryology is taught as a cornerstone subject in medical schools, as well as in biology and zoology programs at both an undergraduate and graduate level.

Ancient Egypt Edit

The study of embryology has a long pedigree. Knowledge of the placenta goes back at least to ancient Egypt, where the placenta was viewed as the seat of the soul. There was even an Egyptian official who held the title Opener of the Kings Placenta. Furthermore, one Egyptian text from the time of Akhenaten claims that a human originates from the egg that grows in women. [4]

Ancient India Edit

A variety of conceptions on embryology appeared in ancient Asia. [5] A more advanced understanding of the embryological process was known from ancient India. Descriptions of the amniotic membrane appear in the Bhagavad Gita, Bhagavata Purana, [6] and the Sushruta Samhita. For example, the Sushruta Samhita claims that an embryo emerges from semen and blood, both of which in turn find their origins in chyle. In the third month, differentiation of body parts such as arms, leg, and head occurs, and this is followed in the fourth month by the development of the heart, thorax, and abdomen. In the sixth month, the hair, bones, sinews, nails, and veins develop, and in the eighth month the vital force (the ojas) is drawn from the mother and to the developing child. The father donates the hard parts of the body to the developing fetus whereas the soft parts come from the mother. Just as with Aristotle, the Sushruta Samhita compares the developing embryo to the clotting of milk into cheese. It claims that conditions of heat result in seven layers of skin being formed around the fetus, just as the creamy layers in cheese form from milk. One of the Upanishads known as the Garbhopanisaḍ states that the embryo is "like water in the first night, in seven nights it is like a bubble, at the end of half a month it becomes a ball. At the end of a month it is hardened, in two months the head is formed". [7] The Indian medical tradition in the Ayurveda also has conceptions of embryology from antiquity. Then Dalhana, a medieval commentator on the Sushruta Samhita, also describes embryological development. Dalhana claims that in the first month, the fetus has a jelly-like form, whereas cold and heat cause a change to hardness during the second month. Limb differentiation occurs in the third to four months and intelligence even later. [8]

Ancient Greece Edit

Pre-Socratic philosophers Edit

Many pre-Socratic philosophers are recorded as having opinions on different aspects of embryology, although there is some bias in the description of their views in later authors such as Aristotle. According to Empedocles (whose views are described by Plutarch in the 1st century AD), who lived in the 5th century BC, the embryo derives and receives its blood from four vessels in all two arteries and two veins. He also held sinews as originating from equal mixtures of earth and air. He further said men begin to form within the first month and are finished within fifty days. Asclepiades agreed that men are formed within fifty days, but he believed that women took a full two months to be fully knit. One observation, variously attributed to either Anaxagoras of Clazomenae or Alcmaeon of Croton, says that the milk produced by mammals is analogous to the white of fowl egg. Diogenes of Apollonia claimed that a mass of flesh forms first, only then followed by the development of bone and nerves. Diogenes was able to recognize that the placenta was a nutritional source for the growing fetus. He further claimed that the development of males took four months, but that the development of females took five months. He did not think the embryo was alive. Alcmaeon also made some contributions, and he is the first person being reported to have practiced dissection. One idea which lasted for a long time, first claimed by Parmenides, was that there was a connection between the right side of the body and the male embryo, and between the left side of the body and the female embryo. According to Democritus and Epicurus, the fetus is nourished at the mouth inside the mother and there are comparable teats that supply this nourishment within the mother's body to the fetus. [9] Discussion on various views regarding how long it takes for specific parts of the embryo to form appear in an anonymous document known as the Nutriment.

Greek discussions on embryology often tried to answer several questions. One question regarded whether only the male had a seed which developed into the embryo within the female womb, or whether both the male and the female had a seed that made a contribution to the developing embryo. The difficulty that one-seed theorists confronted was to explain the maternal resemblance of the progeny. One issue that two-seed theorists confronted was why the female seed was needed if the male already had a seed. One common solution to this problem was to assert that the female seed was either inferior or inactive. Another question to answer was the origin of the seed. One theory to answer this, known as the encephalomyelogenic theory, stated that the seed originated from the brain or and/or bone marrow. Later came pangenesis, which asserted the seed was drawn from the whole body in order to explain the general resemblance in the body of the offspring. Later on hematogenous theory developed which asserted that the seed was drawn from the blood. A third question regarded how or in what form the progeny existed in the seed prior to developing into an embryo and a fetus. According to preformationists, the body of the progeny already existed in a pre-existing but undeveloped form in the seed. Three variants of preformationism were homoiomerous preformationism, anhomoiomerous preformationism, and homuncular preformationism. According to the first, the homoiomerous parts of the body (e.g. humors, bone) already exist pre-formed in the seed. The second held that it was the anhomoiomerous parts that were pre-formed. Finally, the third view held that the whole was already a unified organic thing. Preformationism was not the only view. According to epigenesists, parts of the embryo successively form after conception takes place. [10]

Hippocrates Edit

Some of the most well-known early ideas on embryology come from Hippocrates and the Hippocratic Corpus, where discussion on the embryo is usually given in the context of discussing obstetrics (pregnancy and childbirth). Some of the most relevant Hippocratic texts on embryology include the Regimen on Acute Diseases, On Semen, and On the Development of the Child. Hippocrates claimed that the development of the embryo is put into motion by fire and that nourishment comes from food and breath introduced into the mother. An outer layer of the embryo solidifies, and the fire within consumes humidity which makes way for development of bone and nerve. The fire in the innermost part becomes the belly and air channels are developed in order to route nourishment to it. The enclosed fire also helps form veins and allows for circulation. In this description, Hippocrates aims at describing the causes of development rather than describing what develops. Hippocrates also develops views similar to preformationism, where he claims that all parts of the embryo simultaneously develop. Hippocrates also believed that maternal blood nourishes the embryo. This blood flows and coagulates to help form the flesh of the fetus. This idea was derived from the observation that menstrual blood ceases during pregnancy, which Hippocrates took to imply that it was being redirected to fetal development. Hippocrates also claimed that the flesh differentiates into different organs of the body, and Hippocrates saw as analogous an experiment where a mixture of substances placed into water will differentiate into different layers. Comparing the seed to the embryo, Hippocrates further compared the stalk to the umbilical cord. [11]

Aristotle Edit

Some embryological discussion appears in the writings of Aristotle's predecessor Plato, especially in his Timaeus. One of his views were that the bone marrow acted as the seedbed, and that the soul itself was the seed out of which the embryo developed, though he did not explain how this development proceeded. Scholars also continue to debate the views he held on various other aspects of embryology. [10] However, a much more voluminous discussion on the subject comes from the writings of Aristotle, especially as appears in his On the Generation of Animals. [12] Some ideas related to embryology also appear in his History of Animals, On the Parts of Animals, On Respiration, and On the Motion of Animals. Means by which we know Aristotle studied embryology, and most likely his predecessors as well, was through studying developing embryos taken out from animals as well as aborted and miscarried human embryos. Aristotle believed that the female supplied the matter for the development of the embryo, formed from the menstrual blood whereas the semen that comes from the male shapes that matter. Aristotle's belief that both the male and female made a contribution to the actual fetus goes against some prior beliefs. According to Aeschylus and some Egyptian traditions, the fetus solely develops from the male contribution and that the female womb simply nourishes this growing fetus. On the other hand, the Melanesians held that the fetus is solely a product of the female contribution. Aristotle did not believe there were any external influences on the development of the embryo. Against Hippocrates, Aristotle believed that new parts of the body developed over time rather than all forming immediately and developing from then on. He also considered whether each new part derives from a previously formed part or develops independently of any previously formed part. On the basis that different parts of the body do not resemble each other, he decided in favor of the latter view. He also described development of fetal parts in terms of mechanical and automatic processes. In terms of the development of the embryo, he says it begins in a liquid-like state as the material secreted by the female combines with the semen of the male, and then the surface begins to solidify as it interacts with processes of heating and cooling. The first part of the body to differentiate is the heart, which Aristotle and many of his contemporaries believed was the location of reason and thinking. Aristotle claimed that vessels join to the uterus in order to supply nourishment to the developing fetus. Some of the most solid parts of the fetus cool and, as they lose moisture to heat, turn into nails, horns, hoofs, beaks, etc. Internal heat dries away moisture and forms sinews and bones and the skin results from drying of the flesh. Aristotle also describes the development of birds in eggs at length. He further described embryonic development in dolphins, some sharks, and many other animals. Aristotle singularly wrote more on embryology than any other pre-modern author, and his influence on the subsequent discussion on the subject for many centuries was immense, introducing into the subject forms of classification, a comparative method from various animals, discussion of the development of sexual characteristics, compared the development of the embryo to mechanistic processes, and so forth. [13]

Later Greek embryology Edit

Reportedly, some Stoics claimed that most parts of the body formed at once during embryological development. Some Epicureans claimed that the fetus is nourished by either the amniotic fluid or the blood, and that both male and female supply material to the development of the fetus. According to the writings of Tertullian, Herophilus in the 4th century BC described the ovaries and Fallopian tubes (but not past what was already described by Aristotle) and also dissected some embryos. One advance Herophilus made, against the conceptions of other individuals such as Aristotle, was that the brain was the center of intellect rather than the heart. Though not a part of Greek tradition, in Job 10, the formation of the embryo is likened to the curdling of milk into cheese, as described by Aristotle. Whereas Needham sees this statement in Job as part of the Aristotelian tradition, others see it as evidence that the milk analogy predates the Aristotelian Greek tradition and originates in Jewish circles. [14] In addition, the Wisdom of Solomon (7:2) also has the embryo formed from menstrual blood. Soranus of Ephesus also wrote texts on embryology which went into use for a long time. Some rabbinic texts discuss the embryology of a female Greek writer named Cleopatra, a contemporary of Galen and Soranus, who was said to have claimed that the male fetus is complete in 41 days whereas the female fetus is complete in 81 days. Various other texts of less importance also appear and describe various aspects of embryology, though without making much progress from Aristotle. Plutarch has a chapter in one of his works titled "Whether was before, the hen or egg?" Discussion on embryological tradition also appears in many Neoplatonic traditions. [15]

Next to Aristotle, the most impactful and important Greek writer on biology was Galen of Pergamum, and his works were transmitted throughout the Middle Ages. Galen discusses his understanding of embryology in two of his texts, those being his On the Natural Faculties and his On the Formation of the Foetus. [16] There is an additional text spuriously attributed to Galen known as On the Question of whether the Embryo is an Animal. Among a few of Galen's descriptions, Galen claimed that, by order, the development of parts went from the bones, to nerve, veins, and then other tissues. In general, in the first stage of embryonic development he attributed growth of cartilage, nerve, membrane, ligament, and more. The processes which result in such developments and others include warming, drying, cooling, and combinations thereof. Elsewhere, Galen claimed that there were four main stages in embryological development. The first included an unformed seminal stage, the second a stage where the tria principa forms including the liver, heart, and brain, a third stage where all other parts arise and a fourth stage where all parts become clearly visible. As this development plays out, the form of life of the embryo also moves from that like a plant to that of an animal (where the analogy between the root and umbilical cord is made). Galen claimed that the embryo forms from menstrual blood, by which his experimental analogy was that when you cut the vein of an animal and allow blood to flow out and into some mildly heated water, a sort of coagulation can be observed. He gave detailed descriptions of the position of the umbilical cord relative to other veins. [17]

Patristics Edit

The question of embryology is discussed among a number of patristic authors, largely in terms of theological questions such as whether the fetus has value and/or when it begins to have value. (Although a number of Christian authors continued the classical discussions on the description of the development of the embryo, such as Jacob of Serugh. [18] Passing reference to the embryo also appears in the eighth hymn of Ephrem the Syrian's Paradise Hymns. [19] ) Many patristic treatments of embryology continued in the stream of Greek tradition. [20] The earlier Greek and Roman view that it was not was reversed and all pre-natal infanticide was condemned. Tertullian held that the soul was present from the moment of conception. The Quinisext Council concluded that "we pay no attention to the subtle division as to whether the foetus is formed or unformed". In this time, then, the Roman practice of child exposure came to an end, where unwanted yet birthed children, usually females, were discarded by the parents to die. [21] Other more liberal traditions followed Augustine, who instead viewed that the animation of life began on the 40th day in males and the 80th day in females but not prior. Before the 40th day for men and 80th day for women, the embryo was referred to as the embryo informatus, and after this period was reached, it was referred to as the embryo formatus. The notion originating from the Greeks that the male embryo developed faster remained in various authors until it was experimentally disproven by Andreas Ottomar Goelicke in 1723. [22]

Various patristic literature from backgrounds ranging from Nestorian, Monophysite and Chalcedonian discuss and choose between three different conceptions on the relation between the soul and the embryo. According to one view, the soul pre-exists and enters the embryo at the moment of conception (prohyparxis). According to a second view, the soul enters into existence at the moment of conception (synhyparxis). In a third view, the soul enters into the body after it has been formed (methyparxis). The first option was proposed by Origen, but was increasingly rejected after the fourth century. On the other hand, the other two options were equally accepted after this point. The second position appears to have been proposed as a response to Origen's notion of a pre-existing soul. After the sixth century, the second position was also increasingly seen as Origenist and so rejected on those grounds. The writings of Origen were condemned during the Second Origenist Crises in 553. Those defending prohyparxis usually appealed to the Platonic notion of an eternally moving soul. Those defending the second position also appealed to Plato but rejected his notion on the eternality of the soul. Finally, those appealing to the third position appealed both to Aristotle and scripture. Aristotelian notions included the progression of the development of the soul, from an initial plant-like soul, to a sensitive soul found in animals and allows for movement and perception, and finally the formation of a rational soul which can only be found in the fully-formed human. Furthermore, some scriptural texts were seen as implying the formation of the soul temporally after the formation of the body (namely Genesis 2:7 Exodus 21:22-23 Zachariah 12:1). In the De hominis opificio of Gregory of Nyssa, Aristotle's triparitate notion of the soul was accepted. Gregory also held that the rational soul was present at conception. Theodoret argued based on Genesis 2:7 and Exodus 21:22 that the embryo is only ensouled after the body is fully formed. Based on Exodus 21:22 and Zachariah 12:1, the Monophysite Philoxenus of Mabbug claimed that the soul was created in the body forty days after conception. In his De opificio mundi, the Christian philosopher John Philoponus claimed that the soul is formed after the body. Later still, the author Leontius held that the body and soul were created simultaneously, though it is also possible he held that the soul pre-existed the body. [23]

Some Monophysites and Chalcedonians seemed to have been compelled into accepting synhyparxis in the case of Jesus because of their view that the incarnation of Christ resulted in both one hypostasis and one nature, whereas some Nestorians claimed that Christ, like us, must have had his soul formed after the formation of his body because, per Hebrews 4:15, Christ was like us in all ways but sin. (On the other hand, Leontinus dismissed the relevance of Hebrews 4:15 on the basis that Christ differed from us not only in sinfulness but also conception without semen, making synhyparxis another of Christ's supernatural feats.) They felt comfortable holding this view, under their belief that the human nature of Jesus was separate from the divine hypostasis. Some Nestorians still wondered, however, if the body united with the soul in the moment the soul was created or whether it came with it only later. The Syriac author Babai [ disambiguation needed ] argued for the former on the basis that the latter was hardly better than adoptionism. Maximus the Confessor ridiculed the Aristotelian notion of the development of the soul on the basis that it would make humans parents of both plants and animals. He held to synhyparxis and regarded the other two positions both as incorrect extremes. After the 7th century, Chalcedonian discussion on embryology is slight and the few works that touch on the topic support synhyparxis. But debate among other groups remains lively, still divided on similar sectarian grounds. The patriarch Timothy I argued that the Word first united with the body, and only later with the soul. He cited John 1:1, claiming on its basis that the Word became flesh first, not a human being first. Then, Jacob of Edessa rejected prohyparxis because Origen had defended it and methyparxis because he believed that it made the soul ontologically inferior and as only being made for the body. Then, Moses Bar Kepha claimed, for Christological reasons as a Monophysite, that only synhyparxis was acceptable. He claimed that Genesis 2:7 has no temporal sequence and that Exodus 21:22 regards the formation of the body and not the soul and so is not relevant. To argue against methyparxis, he reasoned that body and soul are both present at death and, because what is at the end must correspond to what is also at the beginning, conception must also have body and soul together. [23]

Embryology in Jewish tradition Edit

Many Jewish authors also discussed notions of embryology, especially as they appear in the Talmud. Much of the embryological data in the Talmud is part of discussions related to the impurity of the mother after childbirth. The embryo was described as the peri habbetten (fruit of the body) and it developed through various stages: (1) golem (formless and rolled-up) (2) shefir meruqqam (embroidered foetus) (3) ubbar (something carried) (4) walad (child) (5) walad shel qayama (viable child) (6) ben she-kallu khadashaw (child whose months have been completed). Some mystical notions regarding embryology appear in the Sefer Yetzirah. The text in the Book of Job relating to the fetus forming by analogy to the curdling of milk into cheese was cited in the Babylonian Talmud and in even greater detail in the Midrash: "When the womb of the woman is full of retained blood which then comes forth to the area of her menstruation, by the will of the Lord comes a drop of white-matter which falls into it: at once the embryo is created. [This can be] compared to milk being put in a vessel: if you add to it some lab-ferment [drug or herb], it coagulates and stands still if not, the milk remains liquid." [14] The Talmud sages held that there were two seeds that participated in the formation of the embryo, one from the male and one from the female, and that their relative proportions determine whether that develops into a male or a female. In the Tractate Nidda, the mother was said to provide a "red-seed" which allows for the development of skin, flesh, hair, and the black part of the eye (pupil), whereas the father provides the "white-seed" which forms the bones, nerves, brain, and the white part of the eye. And finally, God himself was thought to provide the spirit and soul, facial expressions, capacity for hearing and vision, movement, comprehension, and intelligence. Not all strands of Jewish tradition accepted that both the male and female contributed parts to the formation of the fetus. The 13th century medieval commentator Nachmanides, for example, rejected the female contribution. In Tractate Hullin in the Talmud, whether the organs of the child resemble more closely those of the mother or father is said to depend on which one contribute more matter to the embryo depending on the child. Rabbi Ishmael and other sages are said to have disagreed on one matter: they agreed that the male embryo developed on the 41st day, but disagreed on whether this was the case for the female embryo. Some believed that the female embryo was complete later, whereas others held that they were finished at the same time. The only ancient Jewish authors who associated abortion with homicide were Josephus and Philo of Alexandria in the 1st century. Some Talmudic texts discuss magical influences on the development of the embryo, such as one text which claims that if one sleeps on a bed that is pointed to the north-south will have a male child. According to Nachmanides, a child born of a cold drop of semen will be foolish, one born from a warm drop of semen will be passionate and irascible, and one born from a semen drop of medium temperature will be clever and level-headed. Some Talmudic discussions follow from Hippocratic claims that a child born on the eighth month could not survive, whereas others follow Aristotle in claiming that they sometimes could survive. One text even says that survival is possible on the seventh month, but not the eighth. Talmudic embryology, in various aspects, follows Greek discourses especially from Hippocrates and Aristotle, but in other areas, makes novel statements on the subject. [14]

Embryology in the Islamic tradition Edit

Passing reference to embryological notions also appear in the Qur'an (23:12-14), where the development of the embryo proceeds in four stages from drop, to embryo, to fetus, to development of the bone. [24] The notion of clay turning into flesh is seen by some as analogous to a text by Theodoret that describes the same process. [25] The four stages of development in the Qur'an are similar to the four stages of embryological development as described by Galen. In the early 6th century, Sergius of Reshaina devoted himself to the translation of Greek medical texts into Syriac and became the most important figure in this process. Included in his translations were the relevant embryological texts of Galen. Anurshirvan founded a medical school in the southern Mesopotamian city of Gundeshapur, known as the Academy of Gondishapur, which also acted as a medium for the transmission, reception, and development of notions from Greek medicine. These factors helped the transmission of Greek notions on embryology, such as found in Galen, to enter into the Arabian milieu. [26] Very similar embryonic descriptions also appear in the Syriac Jacob of Serugh's letter to the Archdeacon Mar Julian. [18]

Embryological discussions also appear in the Islamic legal tradition. [27]

4.1: Frog Embryology - Biology

  • Accessing Course Resources on the biology shared directory
  • Lab Unit 1:
    • Meiosis & Gametogenesis
    • Cleavage - Gastrula (starfish and sea urchin)
    • Early Frog
    • Lab Exam #5 will be during Final Exam Week.
    • Lecture Unit 1 Resources and Quiz info (More resources are listed under Lab Unit 1)
    • Lecture Unit 2: Links to resources, Quiz Coverage, etc.
    • Lecture Unit 3: Nervous System
    • Lecture Unit 4: Sense Organs, Integument, Circulatory Sys.
    • Lecture Unit 5: Dig. Sys., Resp. Sys., Mesenteries, Branchial arches, UG Sys.
    • Final Exam Comprehensive Lecture Final
    • Final Exam Comprehensive Lecture Final: Format and Coverage
    • Course Resources Applicable to several lecture Units.
    • Email Q&A Forum
    • AH107 Room Schedule and Dr. Ross's Schedule
    • Link to annotated photos of human embryos
    • Life Sciences Dictionary
    • Class Photos Fall 2000
    • News: Human Cloned Embryo (Jan. '02 Sci. Amer. article, full text and illustration) "Cloned early-stage human embryos--and human embryos generated only from eggs, in a process called parthenogenesis--now put therapeutic cloning within reach."
    • Text: Moore, K. L., T. V. Persaud, and Mark G. Torchia. 2013. The Developing Human. Clinically Oriented Embryology. 9th ed. Saunders. ISBN: 978-1437720020 [Amazon]
      • or Moore, K. L. and T. V. Persaud. 2008. 8th ed. ISBN 978-1416037064
      • or (2001) 8th ed. ISBN 978-0138574345 [Amazon]
      • Laboratory Atlas: Wright, Shirley. 2005. Photo Atlas of Developmental Biology 1st ed. Morton. ISBN 9780895826299 [Amazon]
      • Medical Dictionary: Choose a professional-level dictionary, for example: Stedman’'sMedical Dictionary. 28th ed. 2005. Williams and Wilkins. ISBN 978-0781733908
      • Anyone can access the shared volume from any CBU networked Macintosh or PC on campus that can handle file sharing. This includes campus-wide wireless access for your laptop as well as all the PCs in the Computer Center, the Science Building, Buckman, the Library, and Nolan Hall. A person could also connect to this from their CBU dorm room.
      • Map a network drive (Windows):
        • Open Computer and click map network drive on the menu bar [If you don't have a shortcut to Computer on the desktop, use the file folder icon to windows explorer. Then click the help "?" at the upper right of the menu bar and search help for "map network drive". The help box will display a link and instructions.]
        • • At the Map Network Drive dialog box:
          o Drive: (just leave whatever drive letter is shown)
          o Type in Folder: winfile2iology
          o Click this check box: Connect using different credentials
          o Click Finish
        • At the Connect As… dialog box: o Type in User name: cbuyourusername
          o Password: your cbu email password (your Active Directory password)
          o Click OK
          If you are using a shared computer, don't forget to Disconnect the mapped drive when you are finished.
          • Macintosh:
            1. Make sure that you are in finder and not in an application. In the toolbar, the top left hand corner should say "Finder" in bold. If it does not, just click on the desktop background.
            2. Four places over to the right from the word "Finder" in the toolbar it should have the word "Go," click on that and scroll down to the bottom and click on "Connect to server."
            3. A pop up box will appear. In that box you should have a space to type in that says "Server Address." In that space type in the address "smb://" and hit connect. You should now be on the Biology Shared Directory.
        • What's Available : Open the Resources folder for your biology course. Lecture Resources include PowerPoint lecture slides for each course Unit. Lab Resources include required Digital Images and tutorials sorted by lab topic.
        • READ the labels on the microscope slides! The slide boxes contain many different slides. You need to be certain about which slide you need and what you are supposed to notice on each microscope slide.
        • Please take only one microscope slide at a time.
        • Don't park microscope slides on the table tops or in a drawer.
        • Please return microscope slides to the correct box. Please keep slide boxes in order. You may need to consult your lists of microscope slides (Supplement) to determine where the slides go.
        • Please keep your microscope clean. Report any damage to slides or scope immediately.


        • Binocular Compound Microscope Usage(illustrated tutorial) [Except we do NOT wrap the microscope cord around the base.]
          • Oil immersion procedure
          • What are "Primordial germ cells"? Primordial germ cells are diploid cells, capable of mitosis, that migrate into the developing gonads (testes or ovaries) and will later give rise to diploid spermatogonia or oogonia. Primordial germ cells give rise to the germ cell line, the only cells capable of meiosis. ("Spermatogonia" and "oogonia" are diploid cells of the germ cell line.)
          • After DNA replication, each chromosome consists of 2 chromatids (sister chromatids). As an oogonium or spermatogonium prepares for meiosis I, DNA replication occurs during Interphase, before Prophase I.
          • SYNAPSIS is when the 2 members of a homologous pair of chromosomes "come close together." (Synapsis occurs during Prophase I.)
          • The processes that reduce the chromosome number are Meiosis I and the cytokinesis at the end of Meiosis I (at/after telophase I). So, Meiosis I and cytokinesis result in having only one member from each pair of homologous chromosomes in each cell. The cells produced by Meiosis I are secondary spermatocytes (or, in oogenesis, a secondary oocyte and a polar body.)
          • After DNA replication (during interphase I) up until late Anaphase II (after Meiosis II has separated the centromeres [kinetochores] of the indivdual chromosomes), each chromosome consists of 2 sister chromatids.
          • So, what is a tetrad and when is it that we have 4 chromatids close together ? Tetrads are present during Meiosis I, particularly during Prophase I (Synapsis). During Meiosis I, two chromosomes (a homologous pair) find each other. Since each individual chromosome still has 2 chromatids, the homologous pair close together form a "tetrad" (4 chromatids but note that a tetrad is actually made up of 2 chromosomes and each chromsome has 2 chromatids joined by a centromere).
          • Meiosis II makes four spermatids and this is Meiosis II/equational meiosis, right? Yes, each primary spermatocyte will eventually give rise to 4 spermatids , but as a result of Meiosis II and cytokinesis, each secondary spermatocyte gives rise to two spermatids.
          • L eptotene
          • Synaptene ( Z ygotene)
          • P achytene
          • D ipl o tene
          • D i a kenesis

          [Suggested by students in Embryology lab, Fall 2003]

            • PowerPoint Slides prepared by Dr. Ross(full size, updated version is on the shared directory and moodle) [restricted to CBU domain]
            • Tutorial Schoenwolf - PowerPoint slide shows are on the shared directory. Gametogenesis Schoenwolf.ppt : includes photos of sectioned ovary and testis that are NOT in the printed lab manual.
            • Human Ovulation (link to a YouTube video)
            • Human ovulation (animation) Mayo clinic
            • LUMEN (at Loyola Medical School) Human/mammal histology. Annotated images. Testis: labeled slides @ Lumen.
              Ovary: labeled slides @ Lumen

            Oogenesis and Fertilization in Ascaris:

              The Human Reproductive System (Benjamin Cummings)

            • READ the labels on the microscope slides! The slide boxes contain many different slides. You need to be certain about which slide you need and what you are supposed to notice on each microscope slide.
            • Please take only one microscope slide at a time.
            • Don't park microscope slides on the table tops or in a drawer.
            • Please return microscope slides to the correct box. Please keep slide boxes in order. You may need to consult your lists of microscope slides (Supplement) to determine where the slides go.
            • Please keep your microscope clean. Report any damage to slides or scope immediately.
            • When Sperm Meets Egg Acrosome reaction studied in sea urchins (Scientific American)
            • Sea Urchin Development
            • Starfish CleavageGastrulation: Early, late cleavage plus Gastrulation and sections(Rutgers)

            Rana pipiens

            • Amphibian Development Tutorial . (Rutgers) Very good photomicrographs
              Assigned for Embryology Lab Unit 1.
            • Frog CleavageFrog Gastrulation
            • Frog Gastrulation (Xenopus movies)
            • Frog Embryology
            • Grey Crescent: The organizer (Spemann's transplant experiments)
            • Jump to Embryology Page Table of Contents

            Observe the living frog embryos and record your observations on the data form provided in your supplement.

            • 3-4 mm Frog Embryo. Labeled Serial C.S.View all images and study the captions!
            • 5-7 mm Frog Embryo. Labeled Serial C.S.View all images and study the captions!
            • Frog Embryology
            • Tutorial: Schoenwolf PowerPoint slides available on the shared directory. The material for Lab Exam 2 is called "4mm frog Schoenwolf.ppt"
            • Xenopus diagrams of normal development
            • Pictoral Atlas of Xenopus Development Stages
            • Jump to Embryology Page Table of Contents
            • Correction to Schoenwolf: For our purposes, at these early stages of chick development, we'll consider the Sinus Venosus as is an unpaired portion of the Cardiovascular System that is located at the midline. It receives blood from the paired Vitelline Veins. (There is a misleading label on one of the 33-hr. c.s. photographs.)
            • Tutorials: Schoenwolf PowerPoint slide tutorials are on the shared directory. The tutorials for Lab Exam 3 material are: "Chicken 33-hr (Schoenwolf).ppt" "18-hr chick (Schoenwolf).ppt" and " 24-hr chick (Schoenwolf).ppt"
            • Chick 33 hr. w.m. and serial c.s.
            • Chick 33 hr. serial cross sections with interactive labels (Tulane Univ.)
            • 33-36 hr. Labeled WM and selected c.s.
            • Chick N.S. development : Select "View by Systems" then "Development." For our Lab Unit 3, see all the images of 24 hr. and 33 hr. chick (w.m., & c.s.)
            • Early Chick Devel.: Blastula and Gastrulation . Excellent images with captions. (at U Guelph)
            • Chick 24-hr. w.m. and serial c.s.
            • Chick 24 hr. serial cross sections with interactive labels (Tulane Univ.)
            • Which came first, the Chicken or the Egg?
            • Avian Reproduction: Anatomy & the Bird Egg (great info on lots of birds that are not chickens)
            • Just for fun: Peep Research
            • Jump to Embryology Page Table of Contents

            • Students must provide their own disposable gloves (latex or nitrile examination gloves) for the first lab in this unit.
            • Correction to Schoenwolf: 9th ed.: p. 155 fig 4.85 Label #14 (should be #12). The #14 line is pointing to one of the paired Precardinal Veins NOT to one of the paired Dorsal Aortae. [8th ed.: Page 121 Fig. 3.85 Same error]
            • Tutorial: Schoenwolf PowerPoint slide tutorial is available on the shared directory. The material for Lab Exam 4 is called "48-hr Chick (Schoenwolf).ppt"
            • Live chick embryo (video)
            • Chick 48 hr. w.m. and serial c.s.
            • Chick 48 hr. Labeled WM and diagram of optic cup delevopment.
            • 48 hr. serial cross sections with interactive labels (Tulane Univ.)
            • Chicks hatching (video)

            Lab Unit 5
            10 mm Pig

            • Students must provide their own disposable gloves (latex or nitrile examination gloves) for the first lab in this unit.

            Q: How many pigs in a litter?
            A: It depends on the breed of pig. But here's an example: "Current farm data suggests that the herd average for pigs born alive is eleven." Source

            • Date, Time: TBA Both the 9:30 and the 11:00 lab sections will have the Unit 5 lab exam (the lab final) together.
            • Jump to Embryology Page Table of Contents
            • Your Grade in the Embryology Lecture Course will be calculated as the percent of 640 Total Points you have earned for the semester. (The "total" does not include bonus points, so because of Bonus Points on exams and quizzes, it is possible to have an average greater than 100%!)
              In-class Lecture Quizzes #1-7 total 40 points
                [Quiz #8 is a Bonus Quiz, add this score to your lecture points earned]
              • Jump to Embryology Page Table of Contents
                Topics: Digestive System Development and Anatomy
                Format: 50% Diagrams to label 50% Fill-in-the-blank questions.
                Date: TBA.
              • Jump to Embryology Page Table of Contents
              • Lecture Quiz 7: Topics = Sense organs (including inner and middle ear) , Integument (including tooth development) . Detailed diagrams to label. [10 points]
              • Lecture slides are on Moodle and the shared directory

              Lecture Quizzes for Unit 3

              • Lecture Quiz #5: Nervous SystemTopics: First lectures on this unit (lecture + text)
                Format: Multiple Choice. 5 points.
              • Lecture Quiz #6: Nervous System:Topics:
                5 Brain Regions: (Telencephalon, Di-, Mes-, Met-, Myel-)
                Name the brain region that includes a specific cranial nerve nucleus or structure.
                8 Functional Categories (see list, Suppl. p. 63 & slides):
                Name the described functional category.
                12 Cranial Nerves: Give the name and # of the described nerve name the brain region that includes a specified C.N. nucleus.
                Format: Fill-in the blanks. 5 points (10 questions)
              • Please note: A complete understanding of all of the above information is the STARTING POINT in your preparation for lecture Exam #3!DO NOT leave the mastery of these fundamentals to the last minute.
              • Chronolab Human Embryology web site
              • Neuroembryology (Temple Univ.) Features interactive labeled illustrations.
              • Devel. of the spinal cord and spinal nerves:
              • Devel. & Differentiation of the Neural Tube
              • Histogenesis of the N.S. (Brain):
                Includes excellent illustration of Cerebellar histology.
              • Devel of Hypophysis (Temple)
                Scroll down the page to Hypophysis.
              • Histogenesis of the Eye (also for Lect. Unit 4):
                Using Chicken embryo examples.
              • Cranial Nerves
              • Adult Cranial Nerves (Yale Univ. School of Medicine)
              • Cranial Nerves (General info)
              • Human N.S. Development Annotated SEM images
              • Devel. of N.S. & Neuroanatomy (U Wash)
                Human N.S. Development@ Washington
                Well labeled illustrations. Includes a self-quiz.
                Look at each of the following sections:
                Neural tube
                Neural Crest
                Brainstem Nuclei.
                Or, use the following link then follow then choose
                Neuroanat. Syllabus then Development
                • Interactive Neuroanatomy @ Washington
                  Includes embryo and adult neuroanatomy.
                • Jump to Embryology Page Table of Contents

                  Lecture Unit 2 Quizzes:
                    Lecture Quiz # 3 = Date TBA (tentative date listed in syllabus) Topics: Material covered from the start of Unit 2 (

                  1. Essay on the Introduction to Embryology
                  2. Essay on the Historical Review of Embryology
                  3. Essay on the Modern Embryology
                  4. Essay on the Scope of Embryology
                  5. Essay on Gametogenesis
                  6. Essay on the Embryonic Development in Chordates
                  7. Essay on the Fertilisation in Chordates
                  8. Essay on the Stages of Embryogeny

                  Essay # 1. Introduction to Embryology:

                  Embryology (GK., embryon = embryo + logia = discourse) is a study of the origin and development of animals dealing with changes through which a fertilised egg must pass before it assumes the adult state. Fertilisation of an ovum by a spermatozoon results in the formation of a zygote. Development of a single-celled zygote into an adult involves a series of steps or stages resulting in a gradual increase in the complexity of structure.

                  The stages of embryonic development differ in various chordates, yet the chief phases are basically similar in all. The differences are related primarily to the amount and distribution of yolk present in an egg. The inert yolk or vitellin furnishes nourishment for the developing embryo.

                  The yolk also influences on the pattern of cleavage, on the morphogenetic movements of the blastomeres during gastrulation and on the type of development, i.e., indirect with larval forms or direct with juvenile stages.

                  Embryogenesis or embryogeny may be defined as the formation and development of embryos. In fact it includes all the changes by which a fertilised ovum or zygote is transformed into an adult. So long as the developing individual remains in the egg, it is called an embryo. In some lower animals the amount of yolk is less in egg, so that the embryo hatches in earlier stages of development, called a larva.

                  Usually, it is very different in form and structure from the adult. Examples are caterpillars of insects and tadpoles of frogs. The larva undergoes transformation into the adult by the process of metamorphosis. In higher vertebrates like reptiles, birds and mammals, the eggs are richly supplied with yolk. Their embryos continue development until they attain a form resembling the adult. Examples are chicks of birds and foetuses of mammals.

                  Essay # 2. Historical Review of Embryology:

                  Aristotle (384-322 B.C.) was the first Greek philosopher who described the ontogenetic development of chick and many other forms. The doctrines of Aristotle about the development were accepted for a very long time. William Harvey (1578- 1657) and Marcello Malpighi (1628-1694) contributed information on the various stages of the development of chick on the basis of their studies with the help of simple lens. With the discovery of the microscope, Leeuwenhock (1632-1723) described the sperm of man and other mammals.

                  Some ovists namely Swammerdam and Bonnet advocated an extreme form of preformation theory called encasement or “emboitment” theory. This theory holds that successive generations of individual organisms pre-existed one inside the other in the germ cells of the mother. It was estimated that, as many as, 200 million years of human beings were present, already delineated in the ovaries of Eve.

                  Such theories of preformation persisted well in the eighteenth century by which time (in 1759) the German investigator Caspar Friedrich Wolff (1733-1794) offered experimental evidence that no preformed embryo existed in the egg of the chicken. He suggested that during embryonic development the organs formed successively in an epigenetic manner.

                  Wolff advocated that the future embryonic regions of an egg first consist of granules or “globules” (viz., cells or their nuclei) lacking in any arrangement, i.e., these globules do not reveal any resemblance with the form or structure of the future embryo. Only gradually did these “globules” organise into rudiments (germ layers) which, in turn, took on the characteristics of the various organs of the embryo. This method of progressive development from the simpler to the more complex, through the utilisation of building units (globules or cells) is called epigenesis. Today this theory is accepted in a modified form.

                  K.E. Von Baer (1792-1876), the father of modern embryology, was the first embryologist who first of all, presented the embryological data in a coherent form, made various land mark embryological investigations and made certain very important generalisations. He forwarded the germ layer theory which states that “various structures of the body arise from the same germ layers in different species of animals”.

                  His most important generalisation is known as Baer’s law which states that “more general features that are common to all the members of a group of animals are, in the embryo, developed earlier than the more special features which distinguish the various members of the group”.

                  Baer’s law was formulated before the recognition of evolutionary theory, therefore, later on it is reinterpreted in the light of evolutionary theory by Muller and Haeckel (1864) and named as biogenetic law.

                  In 1824, Prevost and Duman described cleavage or segmentation of the egg. Hertwig in 1875 observed the main events taking place in fertilisation of an egg by a sperm. Von Bender (1883) proved that the male and female sex cells contribute the equal number of chromosomes to the fertilised egg.

                  During the last days of Nineteenth and early days of Twentieth century, embryologists like Weismann (1883), Endres (1885), Spermann (1901 and 1903) and Morgan (1908) made experimental and analytical investigations and, thus, a new branch of embryology gave way for the initiation of experimental embryology.

                  In 1883, A. Weismann (1834-1913) suggested convincingly that a child in no way inherits its characters from the bodies of the parents but from the sex cells alone. These germ cells, in turn, acquired their characters directly from the pre-existing germ cells of the same kind.

                  Wilhelm Roux (1850-1924) in 1881, performed a classical experiment which may be viewed as marking the beginning of the science of experimental embryology. He took a frog’s egg at the two cell stage of cleavage and touched one of the two cells with a hot needle, thus, destroying the nucleus.

                  He observed that the uninjured cell continued dividing and developed into what he interpreted to be a one-half blastula, a one-half gastrula, and ultimately a one-half embryo.

                  He, thus, concluded that certain areas of the egg are already destined in the ovary to develop into special region. Thus, the pigmented cytoplasm of animal pole of frog’s unfertilised egg chiefly develops into the head region of the animal, while yolky cytoplasm of vegetal pole of egg forms posterior region.

                  In 1891, the German Scientist Hans Driesch performed experiment on sea urchin eggs, similar to Roux. He suggested that early cleavages of the egg are equational and have a “quantitative division of homogeneous material”, therefore, the blastomeres have equal potentialities and their fate is determined by their position. Development as observed by Driesch in sea urchin eggs was to be called regulative development and the eggs which were capable of performing such regulative development were called regulative eggs.

                  Various operative and chemical procedures have been employed in attempts to analyse the developmental processes leading to or involved in the formation of the blastula, the gastrula and the actively swimming larva. Such an experimental approach of T. Boveri (1910), J. Runnstrom (1928), S. Horstadius (1928) and C.M. Child (1936) on sea urchin eggs has contributed a most important theory the gradient theory.

                  The two gradients are, therefore, the animal gradient with a centre of activity at the animal pole, and the vegetal gradient with a centre of activity at the vegetal pole.

                  C.M. Child (1936), while recognising the physico-chemical nature of the two gradients, proposed the existence of a single physiological or oxidative metabolic gradient in sea urchin egg.

                  It was in 1969 and 1972 that Horstadius and Josefsson succeeded in isolating animalising and vegetalising substances from the mature unfertilised egg and early cleavage stages of sea urchin. Arnold (1976) has suggested that the egg cortex by controlling the displacement of membrane receptors and enzyme systems, modulates metabolism in growth, division and cell surface interaction.

                  In 1924, Spermann and Hilde Mangold published a classical paper providing definitive proof of the organising action of transplanted dorsal lip in the production of secondary embryos, establishing firmly the concept of induction as a basic mechanism in embryonic development. Spermann, thus, has recognised a primary organiser in the form of archenteron in amphibian gastrula and got Nobel Prize of 1935, for such a landmark discovery in experimental embryology.

                  In modern terms induction can be defined as a type of intercellular communication which is required for differentiation, morphogenesis and maintenance. It is also found that during induction some chemical is transmitted from one tissue to the other and this chemical acts on the genes of the cells being induced to develop into a particular manner. What the substance is has not been still determined, but it appears to be a relatively larger molecule.

                  Essay # 3. Modern Embryology:

                  With the discovery of the chromosomes, genes and genetic code, it has become evident that all the properties of any organism are determined by the sequence of the triplets in the DNA molecule. The sequence of the base triplets can directly determine what kind of proteins can be produced by an organism.

                  All the morphological and physiological manifestations of an organism depend on the assortment of proteins, coded for by the hereditary DNA. The modern embryology is heading towards analytical embryology on the basis of the analysis through molecular biology techniques.

                  Essay # 4. Scope of Embryology:

                  Embryology is the most important biological science. It explains the details of the ontogenetic development of an animal from a single fertilised cell. It gives basic information about the physiology, genetics, sex determination, various diseases and organic evolution.

                  Embryology plays a key role in human welfare. It helps in understanding the causes of congenital malformations, cancer, ageing and in improving the breeds of domestic animals, in controlling pests and vectors of diseases and in the formation of test-tube babies.

                  Some of the latest phenomena such as teratogenesis, cancer, animal breeding, test-tube babies and cloning, and pest control are most important fields in animal embryology. With the success in cloning experiments of Ian Wilmut (1996) a new concept of cloning without involving germ cells has originated which is useful for the biological resources. The advantages of cloning plants and animals are numerous.

                  High-yield food plants such as wheat, corn and rice can be selected and abundantly reproduced. Cloning would give animal breeders a tool for exactly reproducing highly desirable animals for example, cloning would make it possible to create 1000 copies of prize dairy cow to help feed growing populations. Endangered species might be saved by cloning numerous replicas of the best of the few remaining individuals.

                  Essay # 5. Gametogenesis:

                  The embryogenesis (embryonic development) of a sexually reproducing multicellular animal is prefaced by the gametogenis, i.e., the formation and ripening of two highly dissimilar and specialised sex-cells or gametes, namely a large-sized, non-motile, nutrient filled cell the ovum or egg and a small-sized,motile,sex-cell, the spermatozoon or sperm, both of which unite and give origin to a diploid zygote.

                  Formation of sex-cell or gametes is termed gametogenesis. It is accompanied by a special type of nuclear division, called meiosis. As a result, the nuclei of gametes formed contain only half or haploid number of chromosomes. When male and female sex-cells (sperms and ova) unite at the time of fertilisation, the resulting cell or zygote again has the full or diploid number of chromosomes.

                  The production of male germ cells, the sperms or spermatozoa occurs in the male gonads, the testes, by a process called spermatogenesis. Each sperm consists of a head, middle piece and tail. It is preferable to call them sperm cells or simply sperms.

                  The production of female germ cells, the ova takes place in female gonads, the ovaries, and the process is called oogenesis. The word ‘egg’ is often loosely used for ova or secondary oocytes. It may be reserved for more complex structures such as the hen’s egg which may even contain early embryonic stages.

                  Essay # 6. Embryonic Development in Chordates:

                  The stages of embryonic development differ in various chordates, yet the chief phases are basically similar in all. The differences are related primarily to the amount and distribution of yolk present in an egg. The inert yolk or vitellin furnishes nourishment for the developing embryo. The yolk also influences on the pattern of cleavage, on the morphogenetic movements of the blastomeres during gastrulation and on the type of development, i.e., indirect with larval forms or direct with juvenile stages.

                  The amount of yolk varies in the eggs of different chordates, it determines the size of the egg and the pattern of early development (cleavage and blastulation, etc.). The eggs are classified according to the distribution of yolk they contain into two main types, namely, isolecithal and telolecithal eggs.

                  A. Isolecithal or homolecithal eggs have very little yolk which is uniformly distributed evenly in the cytoplasm. Such eggs are found in various chordates, e.g., Amphioxus, tunicates and marsupial and eutherian mammals.

                  B. Telolecithal eggs contain a considerable amount of yolk, which has a polarised distribution. Due to its gravity, it is concentrated more in vegetal hemisphere than that of animal hemisphere. Such polarised distribution of yolk is found in mesolacithal and macrolecithal eggs.

                  In fact, in macrolecithal eggs, the amount of yolk is so massive that it almost occupies the whole space of the egg, except a small space at the animal pole where the nucleus or germinal vesicle lies in the form of cap over the yolk.

                  The telolecithal eggs may be either moderately telolecithal (e.g., eggs of Amphibia, Petromyzon and Dipnoi) or highly telolecithal, (e.g., cartilaginous and bony fishes, reptiles, birds and egg-laying mammals). All eggs are enclosed in one or two vitelline membranes.

                  C. Centrolecithal eggs found in insects and some hydrozoa, contain a large amount of yolk concentrated in the centre of the egg surrounded by thin peripheral layer of active cytoplasm.

                  Classification of Eggs on the Basis of Amount of Yolk:

                  1. Microlecithal or oligolecithal eggs are small sized, containing a small amount of yolk. Such eggs are found in Amphioxus, tunicates, and marsupial and eutherian mammals, and also in certain invertebrates such as Hydra and sea urchin.

                  2. Mesolecithal eggs contain moderate amount of yolk, e.g., annelid worms, molluscs, Petromyzontia, Dipnoi and Amphibia.

                  3. Macrolecithal, megalecithal or polylecithal eggs contain massive amount of yolk such as eggs of insects, Myxine, elasmobranch fishes, reptiles, birds and prototherian mammals.

                  The egg or ovum is surrounded by a thin plasma membrane and around it is present a vitelline membrane, which is non-cellular and transparent layer of mucoprotein. It is often much thicker and stronger than the underlying fine plasma membrane. It is differently named in various groups of animals such as chorion in fishes and zona pellucida in reptiles and mammals.

                  A spermatozoon (Gr., sperma = seed + zoon = animal) or male gamete of vertebrates despite its small size is an exceedingly complex cell. It has a head, a middle piece, and a tail, all of these are contained by a continuous plasma membrane, like other living cell.

                  The head has a nucleus invested by a thin layer of cytoplasm which projects in front as a pointed acrosome, both performing two basic functions of the sperm – genetic and activating, respectively. The nucleus occupies most of the space of the sperm head. It is enveloped by a typical double nuclear membrane, which lacks the nuclear pores except the lower part.

                  The nucleus contains only its haploid complement of DNA bound by basic proteins. The nucleus has no nucleolus, RNAs and fluid contents. Acrosome lies anterior to the nucleus and its shape and size varies among different species.

                  It is also bounded by a unit membrane and contains a number of acid hydrolases, such as acid phosphatase, cathepsin, hyaluronidase, etc. In mammals, it contains acrosomin made of hyaluronidase and acrosin (zona lysin).

                  It lies behind the nucleus and connected with the head by a narrow neck. Inside the neck, posterior to the nucleus are present two centrioles, both lie at right angles to the other. The anterior or proximal centriole lies in the depression in the posterior surface of nucleus and forms the mitotic spindle in the egg after fertilisation.

                  The distal centriole or posterior centriole forms the microtubules (axoneme) of the sperm tail (flagellum). It acts as basal body for the flagellum. The distal centriole and the proximal part of the axial filament lie in the middle piece of the spermatozoon. The axial filament of the sperm tail has the same organisation as the axial filament of flagella and cilia.

                  In middle piece the axial filament is surrounded by numerous well developed mitochondria. In mammals, the mitochondria are joined together forming one continuous body twisted spirally around the axial filament.

                  However, in other animals, such as in annelid, Hydroides hexagonus, and in sea urchin, Arbacia punctulata, mitochondria are joined in one or more massive clumps, called mitochondrial bodies forming the bulk of the middle piece. They contain all the respiratory enzymes and are extremely active in oxidative phosphorylation.

                  Around the periphery of middle piece of the sperm is found a condensed layer of cytoplasm that is composed mainly of the microtubules and is called manchette. It also surrounds the posterior part of head of the sperm. At the posterior end of middle piece occurs a dark ring or fibrous thickenings beneath the plasma membrane, forming the boundary between the middle piece and tail. It is called ring centriole or Jensen’s ring.

                  The tail is a long vibratile flagellum containing an axial filament along its whole length and projecting behind the cytoplasm of the tail as an end piece. Tail has two main parts- principal piece and end piece. The principal piece constitutes most of the tail length, consists of a central core, comprising the axial filament.

                  Surrounding this core is a microtubular fibrous tail sheath which some time appears as semicircular ribs oriented perpendicular to the long axis of the filament or as helical coils. In human sperms, out of nine coarse fibres found around axial filament, of the tail two coarse fibres are fused with the surrounding ribs so as to form anterior and posterior columns extending throughout the length of the principal piece.

                  The end piece is merely a short tapering portion of tail containing only the axial filament covered with cytoplasm and plasma membrane.

                  Spermatozoa are discharged from the body floating in a seminal fluid or semen secreted by the seminiferous tubules and accessory reproductive glands. Spermatozoa are always produced in very large numbers.

                  Essay # 7. Fertilisation in Chordates:

                  Fertilisation (L., fertilis = to bear). It is the fusion of two gametes (spermatozoa and ova) and so their nuclei to form a diploid zygote. It activates the egg to form fertilisation membrane outside the egg plasma membrane to start its metabolism and to start its cleavage.

                  During fertilisation process the jelly coats and egg membranes such as vitelline membrane and plasma membrane secrete the fertilizin and the sperms tip secrete antifertilizin, both interact with each other and sperms, thus, agglutinated. It occurs in the female genital tract.

                  The membrane of the acrosomal vesicle of the acrosome and the plasma membrane of the sperm breakdown and the severed edges of the two membranes fuse to form an opening through which the contents of the acrosomal vesicle are released.

                  The inner acrosomal membrane grows into one or many acrosomal tubules which come in contact with the vitelline membrane and plasma membrane. In mammals, the plasma membrane and outer acrosomal membrane break and fuse to give rise to extensive vesiculations and the sperm is possibly phagocytosed by the egg.

                  The acrosome now releases the lytic enzymes or lysins (acrosomin in sea urchins) which help the sperm to penetrate the egg envelopes by liquefying them locally, without affecting the plasma membrane. In mammals including human females, the sperms first penetrate the multiple layers of follicular cells (granulosa cells) which are held together by an adhesive substance hyaluronic acid.

                  The acrosome releases the hyaluronidase and proteolytic enzymes for penetrating the follicle cell layers, corona radiata and zona pellucida. Hyaluronidase is supposed to dissolve the cement between cells of corona radiata. Zona lysins or proteolytic acrosomal enzymes are responsible for the passage of the sperms through zona pellucida.

                  The apical part of sperm plasma membrane (originally the inner acrosomal membrane) extends forward to form an acrosomal tubule. It projects through the egg membranes to reach the egg plasma membrane or oolemma. The shape and size of acrosomal tubule varies with species and is entirely absent in mammals.

                  The tip of acrosomal tubule fuses with egg plasma membrane, while in mammals sperm come in contact with the egg surface by its lateral aspects. After the fusion, the plasma membrane of the egg and tip of acrosomal tubule dissolve at the point of contact. In teleost fishes acrosome is lacking and so the plasma membrane of sperm head fuses directly with the plasma membrane of ovum.

                  After fusion of the both the plasma membranes, the plasma membrane of ovum becomes permeable to sodium, potassium and calcium ions. Calcium is essential for the fertilisation process. pH of egg cytoplasm also increases due to inflow of Na + and outflow of H + ions. Within seconds after membranes contact, changes occur in egg cortex.

                  In bony fishes and frogs, the cortical granules are broken down after sperms’ penetration into the egg cytoplasm and their contents become liquefied and extruded on the surface plasma membrane of the egg. They gradually fill up the perivitelline space in between chorion and egg plasma membrane in bony fishes, and the space between vitelline membrane and egg plasma membrane in frogs.

                  Thus, fertilisation membrane is formed by the rupture of cortical granules outside the plasma membrane. This is due to the cortical reaction stimulated by the penetrating sperm. Fertilisation membrane blocks the entrance of other living sperms.

                  The vitelline membrane or chorion does not transform into fertilisation membrane. In some mammals (e.g., man, rabbit and hamster) the cortical granules burst open and release their contents in the space between egg plasma membrane and zona pellucida. Cortical granules are not found in urodele amphibians and, hence, no fertilisation membrane formation occurs.

                  In most species only one sperm enters the egg and this is called monospermic fertilisation. When many sperms penetrate the single ovum (e.g., in polylecithal eggs of some insects, elasmobranchs, urodeles, reptiles and birds, and also in microlecithal eggs of bryozoans), it is called polyspermic fertilisation. In this case, the genetic material of only one sperm is incorporated in the zygote nucleus, and other sperm nuclei are degenerated.

                  After the penetration of sperm inside the egg cytoplasm, its nucleus moves inward, swells and its chromatin which is very closely packed becomes finely granular. It finally becomes vesicular and is called male pronucleus. Similarly the egg nucleus after second meiotic division undergoes changes and becomes female pronucleus, which swells, increases in volume and becomes vesicular.

                  Later on male and female pronuclei fuse together, that is, the nuclear membrane of both pronuclei are broken at the point of contact and their contents unite into one mass, which is finally bounded by a common nuclear envelope, forming a zygote nucleus. This type of fusion of both pronuclei (male and female) is called amphimixis.

                  Significance of Fertilization:

                  1. The male and female nucleus possess haploid (n) number of chromosomes. The fertilisation restores the specific parental diploid chromosome number.

                  2. Fertilisation brings together the chromosomes and genes from two different parents, resulting into a new genetic recombination.

                  3. Fertilisation activates the egg to undergo cleavage.

                  Types of Fertilization:

                  According to place and nature of fluid media, fertilisation is of two types:

                  A. External Fertilization:

                  When the fertilisation occurs in the aquatic medium outside the body of female, it is called external fertilisation. Aquatic medium may be sea water or freshwater. In marine animals, sexually mature adults shed eggs and sperms freely into the surrounding water. The sperms and eggs are laid in water in astronomical numbers, and also in close proximity.

                  B. Internal Fertilization:

                  In terrestrial forms, where eggs are completely enclosed in impermeable envelopes before being laid such as oviparous animals or where they are retained within maternal body throughout development such as ovo-viviparous and viviparous animals (e.g., elasmobranchs and mammals) the sperms are transmitted internally, i.e., in the body of female, by the intromittent organ of male.

                  In these forms the fertilisation may occur in the lower portion of oviduct (e.g., urodela) or in the upper portion of the oviduct such as salamanders, reptiles, aves and most mammals. In viviparous fishes such as Gambusia affinis and Heterandria formosa, and certain eutherian mammals such as Ericulus, fertilisation occurs in the ovarian haploid follicles.

                  The results of fertilisation are:

                  (a) An activation of the egg to undergo its second maturation division for preparing a haploid female nucleus

                  (b) An introduction of a centriole by the sperm which divides to form two centrioles, since a centriole is lacking in a mature ovum

                  (c) A restoration of a diploid number of chromosomes in the zygote

                  (d) A change in the periphery of the egg which precludes the entry of other sperms

                  (e) Separation of the vitelline membrane from the egg to allow the zygote to rotate.

                  The division of an activated egg (zygote) by a series of mitotic cell divisions into a multitude of cells which become the building units of future organism, is called cleavage or segmentation (Ger., kleiben = to cleave). During cleavage, cells do not grow in size and early cleavage divisions occur synchronously, which is lost during late cleavage.

                  During cleavage, there is no growth in the resulting blastomeres and the total size and volume of the embryo remains the same. The blastomeres do not move so the general shape of the embryo remains the same except the formation of a cavity, the blastocoel in the interior. During cleavage, chemical conversion of reserve food material (yolk, glycogen and ribonucleotides) into active cytoplasm takes place.

                  Thus, a steady increase of respiration occurs throughout cleavage. During cleavage, nucleo-cytoplasmic ratio in cells is reduced, which permits the cells to be more metabolically active, because such nuclei have less cytoplasm to control. Thus, the cleavage converts the egg into a compact mass of cells or blastomeres called morula.

                  The type of cleavage taking place depends largely on the amount of yolk present.

                  Following types of cleavages occur:

                  a. Holoblastic or Total Cleavage:

                  In this type of cleavage, the entire egg divides by each cleavage furrow.

                  It is subdivided into two types:

                  (i) Complete or equal holoblastic cleavage occurs in microlecithal and isolecithal eggs, the entire zygote divides completely to produce a number of almost equal-sized cells, e.g., eutherian mammals, Amphioxus, tunicates.

                  (ii) Unequal holoblastic cleavage occurs in mesolecithal and telolecithal eggs, the zygote divides completely to form unequal-sized blastomeres, i.e., small-sized cells towards the animal pole which has almost no yolk, larger cells towards the yolky vegetal pole, e.g., cyclostomes, elasmobranchs, Dipnoi and Amphibia.

                  b. Meroblastic or Incomplete Cleavage:

                  This occurs in polylecithal eggs in which only the small germinal disc lying at the animal pole consisting of clear cytoplasm and a nucleus, undergoes a series of incomplete divisions forming an area of cells at the animal pole, the large yolky portion beneath the germinal disc remains unsegmented, e.g., toleosts, reptiles, birds and egg-laying mammals. Here the germinal disc is of disc-shape, so the cleavage is also called discoidal.

                  c. Superficial Cleavage:

                  This type of incomplete cleavage is found in centrolecithal eggs, e.g., insects and many arthropods. The nucleus lying in the centre of the egg yolk surrounded by an island of cytoplasm undergoes cleavage, and each nuclei is surrounded by small amount of cytoplasm.

                  They later move towards the periphery in the peripheral cytoplasm. Here their cytoplasm fuses with the peripheral cytoplasm. Later the peripheral cytoplasm becomes subdivided by furrows extending inward from the surface, thus, a layer of peripheral or superficial cells is formed which surrounds the central undivided yolk.

                  The pattern of cleavage due to organisation of egg may be of following types:

                  i. Radial Cleavage:

                  When successive cleavages extend through the egg, at right angles to one another and the resulting blastomeres become symmetrically arranged around the polar axis. Such type of cleavage is called radial cleavage, and is found in echinoderms (e.g., Synapta and Paracentrotus, etc.).

                  ii. Biradial Cleavage:

                  When first three cleavage planes are not arranged at right angles to each other, it is called biradial cleavage, e.g., Acoela like Polychoerus and Ctenophora.

                  iii. Spiral Cleavage:

                  The rotational movement of cells around the egg axis during cleavage is due to spiral cleavage. The spiral cleavage results due to oblique positions of mitotic spindles in the blastomeres. Thus, it is also called oblique cleavage. In successive cleavages, the rotational movements alternate in clockwise direction or anticlockwise direction. It is found in Turbellaria, Nematoda, Rotifera, Annelida and molluscs except cephalopods.

                  iv. Bilateral Cleavage:

                  In this type of cleavage the mitotic spindles and cleavage planes remain bilaterally arranged with reference to a plane of symmetry which coincides with the median plane of the embryo. It is found in Tunicata, Amphioxus, Amphibia and higher mammals.

                  v. Determinate and Indeterminate Cleavage:

                  The cleavage in nematodes is of a special type of bilateral cleavage in which definite blastomeres give rise to specific parts of the embryo. This type of cleavage is called determinate or mosaic cleavage. In vertebrates, the plane of cleavage is less rigid, the cleavage pattern has no definite relation to the embryo.

                  This type of cleavage is called indeterminate or regulative and is found in echinoderms, Balanoglossus, coelenterates and amphibians. A first cleavage blastomere of a sea urchin or an amphibian or a mammal, when isolated can alter its usual destiny and develop into a perfect (but small) embryo. Similarly, when two fertilised eggs, made to adhere like a two-cell stage, they produce a single giant embryo. This is regulative development.

                  Essay # 8. Stages of Embryogeny:

                  During early cleavages, the blastomeres tend to assume a spherical shape and their mutual pressure flattens the surfaces of the blastomeres in contact with each other, but their free surfaces remain spherical.

                  Thus, cleavage process develops a multicellular body with loosely arranged blastomeres with in fertilisation membrane, called morula (Latin word for mulberry) resembling mulberry, e.g., amphibian and coelenterates. In macrolecithal eggs, morula is a cellular flattened disc at the animal pole.

                  As cleavage proceeds the cells increase in number but become smaller. The cells withdraw from the centre and arrange themselves towards the surface to form a true epithelium, which may be single cell thick as in Amphioxus, echinoderms, etc., or many cell thick as in most vertebrates.

                  Due to rearrangement of cells to form the epithelium or blastoderm a fluid-filled space or blastocoel or segmentation cavity is formed. This stage is called blastula and the process of formation is called blastulation.

                  Types of Blastulae:

                  i. Coeloblastula:

                  It is in the form of a hollow sphere formed of a single layer of blastoderm and the blastocoel is filled with mucopolysaccharides. Examples, echinoderms and Amphioxus.

                  ii. Stereoblastula:

                  In spirally cleaving eggs of annelids, molluscs, nemerteans and some planarians, blastula is solid, having no blastocoelic cavity. In them micromeres accumulate as cluster of cells over macromeres of vegetal hemisphere.

                  iii. Periblastula or Superficial Blastula:

                  In superficially cleaving eggs of insects, the blastocoelic cavity is not found. The central yolk is surrounded by peripherally arranged cells.

                  iv. Discoblastula:

                  In large yolky eggs of fishes, reptiles and birds discoblastula is found. It is a small multilayered flat disc separated from the yolk by a narrow subgerminal cavity.

                  v. Amphiblastula:

                  It is found in amphibians. The blastula contains micromeres in the animal hemisphere and macromeres in the vegetal hemisphere, and a small fluid-filled eccentric blastocoel in the animal hemisphere.

                  It is found in mammals. Cleavage is regular and a small cavity appears inside the dividing cells, which gradually increases in volume. This is the blastocoel. The cells surrounding the blastocoel are the trophoblast cells or nutritive cells and an inner cell mass of formative cells displaced to one pole of the blastocyst.

                  A rearrangement of the cells of the blastula occurs in which some cells are differentiated and come to lie inside, while the other cells enclose them, this stage is gastrula and the processes converting the blastula into a gastrula are known as gastrulation. Gastrulation process (morphogenetic movements of cells) converts a simple one-layered blastula into a two-layered (e.g., Amphioxus) or a three-layered (e.g., all vertebrates) gastrula (Gr., gaster = stomach or gut).

                  The single layer of blastula is called blastoderm, ectoblast or proctoderm. The three layers (ectoderm, mesoderm and endoderm) are called germinal layers. The blastocoel is generally obliterated and the inner layer of cells (endoderm) of the gastrula encloses a new cavity called archenteron which opens on one side to the exterior by a blastopore. During gastrulation embryo acquires antero-posterior polarity and bilateral symmetry.

                  After gastrulation the continuous masses of cells of the three germ layers split up into smaller groups of cells, called primary organ rudiments, each of which produces a certain organ or part of the animal body. These organ rudiments further develop simple organs and parts and, thus, embryo develops into either larval form or a miniature adult. Thus, the formation of organs from the germ layers is called organogenesis.

                  Derivatives of Germ Layers:

                  The ectoderm forms a neural tube which gives rise to the brain, spinal cord, and nerves. The forebrain forms the retina, and part of the iris. The ectoderm forms the lens, conjunctiva, and a part of the cornea, the membranous labyrinth and the lining of the nose.

                  In fishes and aquatic amphibians, the sensory parts of the lateral line system arise from the ectoderm. The neural crest cells lying between the outer ectoderm and on both sides of the neural tube give rise to ganglia of the spinal nerves and autonomic nervous system, the neurilemma of peripheral nerves, chromatophores of the skin, some neural crest cells give rise to mesenchyme which produces the visceral arches, and some neural crest cells wander inwards and form the suprarenal gland near the kidneys, but in mammals they form the medulla of adrenal glands.

                  Supporting part of the central nervous system called neuroglia is derived from the neural tube. The ectoderm forms the epidermis of the skin and many epidermal derivatives, such as skin glands, epidermal scales, nails, claws, hoofs, horns, feathers and hairs.

                  Ectodermal invaginations form the stomodaeum and proctodaeum which meets the archenteron, the ectoderm of the stomodaeum forms the lining of the mouth and lips, glands of buccal cavity, enamel of teeth, covering of tongue, and anterior and intermediate lobes of the pituitary gland (the posterior lobe of the pituitary is formed from the forebrain).

                  The ectoderm of proctodaeum forms the lining of the cloaca and some anal and cloacal glands. From the dorsal side of the forebrain one or two evaginations take place, the anterior one is an eye-like parietal body which is present in lower forms only, the posterior one is the pineal body found in all vertebrates.

                  The archenteron is formed from endoderm, it becomes the lining of the adult alimentary canal, except in the buccal cavity and cloaca. Two outgrowths of the digestive tract form the liver and pancreas, the endoderm forming their epithelial lining only, and also of the gall bladder and bile duct.

                  From the pharynx, the endoderm pushes out to form several pairs of pharyngeal pouches. In cyclostomes, fishes, and amphibians, the pharyngeal pouches meet the ectoderm to form gill-clefts which open to the exterior. In amniotes, the pharyngeal pouches do not perforate to the exterior, in tetrapoda, the first pair is modified to form the cavity of the middle ear and Eustachian tube.

                  An evagination of the pharynx along with some pharyngeal pouches forms a thyroid gland. In air-breathing vertebrates the endoderm of pharynx forms the lining of the larynx, trachea, and lungs. Endoderm of some pharyngeal pouches form part of the tonsils, thymus, parathyroid glands and ultimobranchial bodies.

                  In amniotes, the archenteron forms a large bag, the allantois, its lining is endodermal. Endoderm cells of the archenteron grow out in embryos developing from polylecithal eggs to form the lining of the yolk sac to enclose the yolk, the yolk sac disappears in the adult. It must be noted that organs arising from the archenteron have only their lining and epithelial cells formed from endoderm, the supporting tissues of these organs are mesodermal.

                  The mesoderm becomes differentiated into three major parts- a dorsal epimere which is segmented, a median mesomere, and a ventral hypomere. Further development of mesoderm forms mesenchyme which is not a germinal layer but a primitive kind of embryonic connective tissue with branching cells forming a network. Nearly all mesenchyme comes from mesoderm though other germinal layers may also contribute to its formation.

                  The epimere is differentiated into sclerotome, dermatome, and myotome. The middle parts of epimeres form mesenchyme which gathers around the neural tube and notochord to form the sclerotome. The mesenchymatous sclerotome forms the vertebral column.

                  The dermatome transforms into mesenchyme which migrates to lie below the ectoderm and gives rise to the dermis of the skin. The remaining portion of the epimere is called myotome, the adjacent myotomes are separated by myocommata which are connective tissue partitions. The myotomes of the two sides grow down between the skin and somatic layer of mesoderm to meet midventrally, they give rise (with some exceptions) to voluntary muscles of the body and body wall.

                  (ii) Mesomere forms the urogenital organs and their ducts, the terminal parts of these ducts may have ectodermal or endodermal lining.

                  (iii) Hypomere splits into somatic and splanchnic layers of mesoderm enclosing the coelom. The splanchnic layer forms mesenchyme which gives rise to involuntary muscles and connective tissue of the alimentary canal and of the organs formed as outgrowths of the archenteron.

                  The splanchnic mesoderm forms the heart. The remainder of the splanchnic mesoderm together with the somatic mesoderm forms the lining of the coelom, pericardium and lung pleura or peritoneum. Splanchnic mesoderm also forms the mesenteries and omenta.

                  (iv) Mesenchyme (Gk., mesos = middle + enchyma = infusion) gives rise to all the connective tissue, blood vessels, lymph vessels, lymph nodes, blood corpuscles, all involuntary muscles, parts of the eye, dentine of teeth, and to cartilage and bones of the entire skeleton, except the vertebral column. It is claimed that voluntary muscles of limbs are formed from mesenchyme and not from myotomes.


                  • an opening (the blastopore) that will be the future anus
                  • a cluster of cells that develops into the Spemann organizer (named after one of the German embryologists who discovered its remarkable inductive properties).
                  • ectoderm
                  • mesoderm
                  • endoderm
                  • develop into the notochord, which is the precursor of the backbone
                  • induce the ectoderm lying above it to begin to form neural tissue instead of skin.
                    • This ectoderm grows up into two longitudinal folds, forming the neural folds stage.
                    • In time the lips of the folds fuse to form the neural tube.
                    • The neural tube eventually develops into the brain and spinal cord.


                    Natural nest temperatures and egg clutch parameters

                    The temperatures of two G. victoriana nests ranged between 5.0 and 17.8°C with a mean of 10.9±0.03°C. Clutches ranged in size from 56 to 232 eggs with a mean of 113±11 eggs (N=18). The proportion of viable embryos in each clutch averaged 89.7±5.3%(N=19) and the mean ovum diameter (embryos <2 days old) was 2.27±0.21 mm (N=180 eggs).

                    The relationship between incubation water potential and (A) egg mass, and(B) capsule and perivitelline membrane diameters of stage 26 G. victoriana eggs. Error bars are ±1 s.e.m. Values for eggs raised above pure water (vapour water potential, ψv=0 kPa vapour pressure of water, PH2O=1.4013 kPa) are shown by dotted lines. * Significant difference from values for eggs raised above pure water. † Significant linear regression. Open symbols are the values for eggs raised in shallow pure water(ψπ=0 kPa). Values are means ±1 s.e.m. Nvalues for each treatment are as listed for the variable `Total length' in Table 3.

                    The relationship between incubation water potential and (A) egg mass, and(B) capsule and perivitelline membrane diameters of stage 26 G. victoriana eggs. Error bars are ±1 s.e.m. Values for eggs raised above pure water (vapour water potential, ψv=0 kPa vapour pressure of water, PH2O=1.4013 kPa) are shown by dotted lines. * Significant difference from values for eggs raised above pure water. † Significant linear regression. Open symbols are the values for eggs raised in shallow pure water(ψπ=0 kPa). Values are means ±1 s.e.m. Nvalues for each treatment are as listed for the variable `Total length' in Table 3.

                    Effects of incubation treatments on egg and embryo mass and morphology

                    Comparison of water vapour treatments

                    Egg mass decreased linearly with decreasing ψv(Table 2 Fig. 1). The diameters of the perivitelline membrane and capsule, and consequently capsule thickness, also decreased with decreasing ψv(Table 2 Figs 1 and 2). Eggs from atmospheres below–493 kPa had significantly smaller perivitelline diameters and thinner capsules than control eggs, while eggs in atmospheres below –206 kPa had significantly smaller capsule diameters (Figs 1 and 2).

                    Linear regressions describing the relationship between water potential and a range of parameters measured in embryos from the vapour pressure treatments

                    Variable . a . b . Sa . Sb . Syx . R 2 . P .
                    Egg mass (g) 2.17×10 –2 1.38×10 –5 5.48×10 –4 1.58×10 –6 1.04 0.94 <0.001 *
                    Perivitelline membrane diameter (mm) 3.16 7.19×10 –4 2.19×10 –2 9.35×10 –5 7.42×10 –1 0.92 0.001 *
                    Capsule diameter (mm) 3.64 1.08×10 –3 4.11×10 –2 1.55×10 –4 1.10 0.91 0.001 *
                    Capsule thickness (mm) 5.03×10 –1 4.56×10 –4 3.78×10 –2 1.17×10 –4 1.91 0.75 0.011 *
                    Whole (embryo+gut) wet mass (g) 1.27×10 –2 6.90×10 –7 3.08×10 –4 1.38×10 –6 1.32 0.05 0.640
                    Whole (embryo+gut) dry mass (g) 2.21×10 –3 1.70×10 –7 2.25×10 –5 1.00×10 –7 5.64×10 –1 0.38 0.142
                    Embryo dry mass (g) 1.15×10 –3 3.60×10 –7 2.84×10 –5 1.10×10 –7 9.90×10 –1 0.68 0.022 *
                    Gut dry mass (g) 1.05×10 –3 –2.30×10 –7 3.26×10 –5 1.40×10 –7 1.31 0.35 0.165
                    Body height (mm) 2.17 –4.84×10 –4 3.64×10 –2 2.26×10 –4 1.46 0.48 0.085
                    Fin height (mm) 1.79 2.61×10 –4 2.67×10 –2 9.94×10 –5 7.81×10 –2 0.58 0.047 *
                    Tail muscle height (mm) 1.17 –3.86×10 –5 2.58×10 –2 1.78×10 –4 1.76 0.00 0.837
                    Total length (mm) 9.55 2.84×10 –3 1.06×10 –1 4.29×10 –4 7.79 0.90 0.001 *
                    Tail length (mm) 6.44 2.64×10 –3 6.73×10 –2 2.90×10 –4 5.73×10 –1 0.94 <0.001 *
                    Snout–vent length (mm) 3.15 3.66×10 –3 4.78×10 –1 2.11×10 –4 1.40 0.38 0.143
                    Body width (mm) 2.32 –2.42×10 –4 2.78×10 –2 1.60×10 –4 1.03 0.32 0.189
                    Tail width (mm) 8.58×10 –1 –1.23×10 –4 1.05×10 –2 8.19×10 –5 1.08 0.31 0.193
                    Arcsine square root of % embryo total dry mass 8.07×10 –1 1.33×10 –4 1.23×10 –2 4.58×10 –5 1.41 0.63 0.034 *
                    Arcsine square root of % gut of total dry mass 4.79×10 –1 –1.32×10 –4 1.22×10 –2 4.55×10 –5 1.42 0.63 0.034 *
                    Variable . a . b . Sa . Sb . Syx . R 2 . P .
                    Egg mass (g) 2.17×10 –2 1.38×10 –5 5.48×10 –4 1.58×10 –6 1.04 0.94 <0.001 *
                    Perivitelline membrane diameter (mm) 3.16 7.19×10 –4 2.19×10 –2 9.35×10 –5 7.42×10 –1 0.92 0.001 *
                    Capsule diameter (mm) 3.64 1.08×10 –3 4.11×10 –2 1.55×10 –4 1.10 0.91 0.001 *
                    Capsule thickness (mm) 5.03×10 –1 4.56×10 –4 3.78×10 –2 1.17×10 –4 1.91 0.75 0.011 *
                    Whole (embryo+gut) wet mass (g) 1.27×10 –2 6.90×10 –7 3.08×10 –4 1.38×10 –6 1.32 0.05 0.640
                    Whole (embryo+gut) dry mass (g) 2.21×10 –3 1.70×10 –7 2.25×10 –5 1.00×10 –7 5.64×10 –1 0.38 0.142
                    Embryo dry mass (g) 1.15×10 –3 3.60×10 –7 2.84×10 –5 1.10×10 –7 9.90×10 –1 0.68 0.022 *
                    Gut dry mass (g) 1.05×10 –3 –2.30×10 –7 3.26×10 –5 1.40×10 –7 1.31 0.35 0.165
                    Body height (mm) 2.17 –4.84×10 –4 3.64×10 –2 2.26×10 –4 1.46 0.48 0.085
                    Fin height (mm) 1.79 2.61×10 –4 2.67×10 –2 9.94×10 –5 7.81×10 –2 0.58 0.047 *
                    Tail muscle height (mm) 1.17 –3.86×10 –5 2.58×10 –2 1.78×10 –4 1.76 0.00 0.837
                    Total length (mm) 9.55 2.84×10 –3 1.06×10 –1 4.29×10 –4 7.79 0.90 0.001 *
                    Tail length (mm) 6.44 2.64×10 –3 6.73×10 –2 2.90×10 –4 5.73×10 –1 0.94 <0.001 *
                    Snout–vent length (mm) 3.15 3.66×10 –3 4.78×10 –1 2.11×10 –4 1.40 0.38 0.143
                    Body width (mm) 2.32 –2.42×10 –4 2.78×10 –2 1.60×10 –4 1.03 0.32 0.189
                    Tail width (mm) 8.58×10 –1 –1.23×10 –4 1.05×10 –2 8.19×10 –5 1.08 0.31 0.193
                    Arcsine square root of % embryo total dry mass 8.07×10 –1 1.33×10 –4 1.23×10 –2 4.58×10 –5 1.41 0.63 0.034 *
                    Arcsine square root of % gut of total dry mass 4.79×10 –1 –1.32×10 –4 1.22×10 –2 4.55×10 –5 1.42 0.63 0.034 *

                    Linear regression coefficients in the form y=a+bx, where y is the variable of concern and x is the water potential, Sa is the standard error of a, Sb is the standard error of b, Syx is the standard error of estimate and R 2 is the coefficient of determination. * Significant regressions (P<0.05)

                    There were no differences in whole (embryo+gut) wet or whole (embryo+gut)dry mass between the various ψv treatments(Table 2 Fig. 3), but dry gut-free embryo mass decreased with decreasing ψv. The relative proportion of dry gut to dry body tissue was influenced by ψv,with embryos reared in the driest conditions assimilating less yolk than better hydrated embryos. Total embryo length decreased with decreasingψ v (Tables 2 and 3 Fig. 2). Stage 26 embryos raised at ψv=0 kPa were longer than those embryos raised in atmospheres below ψv=–493 kPa. Tail length was shorter with decreasing ψv, but snout–vent length was only significantly different between embryos raised at ψv=0 and embryos raised at –105 kPa and –493 kPa (Tables 2 and 3 Fig. 2). Body height only differed between embryos from ψv=0 kPa and atψ v=–533 kPa (Table 3). Fin height decreased with decreasing ψv. There was no effect of incubation ψv on tail muscle height or tail width (Tables 2 and 3 Fig. 2). Fixed embryo dimensions were 95% of live embryo dimensions (P=0.001, Student's t-test).

                    Morphology of G. victoriana embryos incubated to stage 26 at a range of water potentials

                    Water Potential (kPa) . Total length (mm) . Snout–vent length (mm) . Tail length (mm) . Body height (mm) . Fin height (mm) . Tail muscle height (mm) . Body width (mm) . Tail width (mm) .
                    0 11.20±0.35 * (14) 3.84±0.05 * (14) 7.37±0.32 (14) 2.08±0.04 (14) 2.27±0.10 * (14) 1.20±0.02 (14) 2.30±0.03 (14) 0.82±0.02 (14)
                    0 9.84±0.37 (20) 3.26±0.06 (20) 6.58±0.32 (20) 2.19±0.04 (18) 1.84±0.08 (17) 1.23±0.03 (18) 2.37±0.07 (20) 0.88±0.02 (20)
                    –22 9.67±0.23 (20) 3.16±0.07 (20) 6.51±0.18 (20) 2.10±0.04 (19) 1.79±0.07 (19) 1.15±0.02 (19) 2.31±0.04 (20) 0.86±0.01 (20)
                    –52 9.26±0.25 (17) 3.09±0.06 (17) 6.17±0.23 (17) 2.32±0.06 (15) 1.81±0.06 (13) 1.21±0.03 (15) 2.35±0.06 (17) 0.87±0.03 (16)
                    –105 9.05±0.20 (17) 3.02±0.06 * (17) 6.03±0.18 (17) 2.19±0.05 (17) 1.72±0.05 (14) 1.12±0.03 (17) 2.26±0.06 (17) 0.85±0.02 (17)
                    –206 9.03±0.40 (13) 3.12±0.10 (13) 5.90±0.32 (13) 2.25±0.12 (10) 1.64±0.09 (10) 1.29±0.10 (10) 2.44±0.06 (12) 1.10±0.15 * (11)
                    –493 7.95±0.48 * (10) 2.83±0.12 * (10) 5.12±0.45 * (10) 2.29±0.11 (8) 1.68±0.05 (8) 1.16±0.06 (9) 2.35±0.13 (8) 0.94±0.05 (8)
                    –533 8.15±0.27 * (9) 3.06±0.08 (9) 5.09±0.25 * (9) 2.52±0.10 * (8) 1.65±0.10 (7) 1.31±0.09 (9) 2.45±0.10 (7) 0.93±0.05 (7)
                    Water Potential (kPa) . Total length (mm) . Snout–vent length (mm) . Tail length (mm) . Body height (mm) . Fin height (mm) . Tail muscle height (mm) . Body width (mm) . Tail width (mm) .
                    0 11.20±0.35 * (14) 3.84±0.05 * (14) 7.37±0.32 (14) 2.08±0.04 (14) 2.27±0.10 * (14) 1.20±0.02 (14) 2.30±0.03 (14) 0.82±0.02 (14)
                    0 9.84±0.37 (20) 3.26±0.06 (20) 6.58±0.32 (20) 2.19±0.04 (18) 1.84±0.08 (17) 1.23±0.03 (18) 2.37±0.07 (20) 0.88±0.02 (20)
                    –22 9.67±0.23 (20) 3.16±0.07 (20) 6.51±0.18 (20) 2.10±0.04 (19) 1.79±0.07 (19) 1.15±0.02 (19) 2.31±0.04 (20) 0.86±0.01 (20)
                    –52 9.26±0.25 (17) 3.09±0.06 (17) 6.17±0.23 (17) 2.32±0.06 (15) 1.81±0.06 (13) 1.21±0.03 (15) 2.35±0.06 (17) 0.87±0.03 (16)
                    –105 9.05±0.20 (17) 3.02±0.06 * (17) 6.03±0.18 (17) 2.19±0.05 (17) 1.72±0.05 (14) 1.12±0.03 (17) 2.26±0.06 (17) 0.85±0.02 (17)
                    –206 9.03±0.40 (13) 3.12±0.10 (13) 5.90±0.32 (13) 2.25±0.12 (10) 1.64±0.09 (10) 1.29±0.10 (10) 2.44±0.06 (12) 1.10±0.15 * (11)
                    –493 7.95±0.48 * (10) 2.83±0.12 * (10) 5.12±0.45 * (10) 2.29±0.11 (8) 1.68±0.05 (8) 1.16±0.06 (9) 2.35±0.13 (8) 0.94±0.05 (8)
                    –533 8.15±0.27 * (9) 3.06±0.08 (9) 5.09±0.25 * (9) 2.52±0.10 * (8) 1.65±0.10 (7) 1.31±0.09 (9) 2.45±0.10 (7) 0.93±0.05 (7)

                    Values are means ± s.e.m. (N) N values vary because some dimensions could not be accurately measured from the digital images. * Significant difference from the control 0 kPa vapour treatment value (P<0.05)

                    Examples of unhatched G. victoriana embryos and lateral and dorsal views of hatched larvae at Gosner stage 26. The scale bar is 2.5 mm.

                    Examples of unhatched G. victoriana embryos and lateral and dorsal views of hatched larvae at Gosner stage 26. The scale bar is 2.5 mm.

                    Comparison of liquid and vapour phase

                    There were significant differences between G. victoriana eggs incubated on a wet substrate (ψπ=0 kPa) and the control(ψv=0 kPa) eggs. Eggs incubated on a wet substrate were 4.8 times heavier than control eggs, the perivitelline membrane and capsule diameters were significantly greater and the jelly capsule was thicker(Fig. 1). Stage 26 embryos raised at ψπ=0 kPa were 18% heavier and 14% longer than embryos reared at ψv=0 kPa(Fig. 3). The greater length ofψ π=0 kPa embryos was due to both greater snout–vent length and greater tail length. Fin height of embryos raised atψ π=0 kPa was 23% higher than for control(ψv=0 kPa) embryos (Table 3 Fig. 2).

                    The relationship between incubation water potential and (A) whole(embryo+gut) embryo wet mass, whole (embryo+gut) embryo dry mass, dry gut-free embryo mass and dry gut mass and (B) the proportion of dry mass in gut and body of stage 26 G. victoriana embryos. Values for eggs raised above pure water (ψv=0 kPa, PH2O=1.4013 kPa) are shown by dotted lines. * Significant difference from values for eggs raised above pure water. † Significant linear regression. Open symbols are the values for eggs raised in shallow pure water (ψπ=0 kPa). Values are means ±1 s.e.m. N values for each treatment are as listed for the variable `Total length' in Table 3.

                    The relationship between incubation water potential and (A) whole(embryo+gut) embryo wet mass, whole (embryo+gut) embryo dry mass, dry gut-free embryo mass and dry gut mass and (B) the proportion of dry mass in gut and body of stage 26 G. victoriana embryos. Values for eggs raised above pure water (ψv=0 kPa, PH2O=1.4013 kPa) are shown by dotted lines. * Significant difference from values for eggs raised above pure water. † Significant linear regression. Open symbols are the values for eggs raised in shallow pure water (ψπ=0 kPa). Values are means ±1 s.e.m. N values for each treatment are as listed for the variable `Total length' in Table 3.

                    Osmolality of perivitelline and interstitial fluid

                    The osmolality of perivitelline fluid measured in unhatched stage 26 embryos was 10±2 mosmol kg –1 , which was equivalent to a ψπ of –24±2 kPa (N=16). The osmolality of interstitial fluid from the tails of unhatched stage 26 embryos was 179±2 mosmol kg –1 (N=3), equivalent to aψ π of –399±15 kPa. The osmolality of the heads and residual yolk material was 194±1 mosmol kg –1 (N=3), equivalent to a ψπ of –441±2 kPa. On average, the osmolality of the entire embryo (tail+head) was 186±2 mosmol kg –1 , equivalent to aψ π of –424±5 kPa.

                    Rate of oxygen consumption

                    There were no differences in the dry mass-specific rate of oxygen consumption (O2,μlh –1 mg –1 ) for eggs at stage 26 that had been raised at different ψv values or between the control and 0 kPa ψπ embryos (Fig. 4). The pre-hatching stage 26 O2 across all treatments averaged 0.92±0.09 μlh –1 mg –1 .

                    Survival rates and hatching success

                    There was no effect of ψv on the percentage of embryos surviving to hatching stage 26 (P=0.302) with survival averaging 37.12±7.10% (N=42 containers) across the ψvtreatments. There was also no effect of ψv on the number of embryos that were able to hatch when flooded (P=0.067), which averaged 74.01±5.23% (N=42 containers). The percentage ofψ v=0 kPa versus ψπ=0 kPa embryos surviving to hatching was not different (P=0.17), averaging 39.17±8.21% and 40.00±2.24%, respectively. Similarly, the hatching rates of embryos raised at ψv=0 kPa versusψ π=0 kPa were also not different (P=0.994),averaging 74.81±9.96% and 85.40±2.30%, respectively.

                    The relationship between incubation water potential and dry mass-specific rate of oxygen consumption (μl h –1 mg –1 )of unhatched stage 26 G. victoriana embryos. Values for eggs raised above pure water (ψv=0 kPa, PH2O=1.4013 kPa) are shown by the dotted line. Open symbol shows the value for eggs raised in shallow pure water(ψπ=0 kPa). No statistical significance between means was observed. Values are means ±1 s.e.m. N values for each treatment are as listed for the variable `Total length' in Table 3.

                    The relationship between incubation water potential and dry mass-specific rate of oxygen consumption (μl h –1 mg –1 )of unhatched stage 26 G. victoriana embryos. Values for eggs raised above pure water (ψv=0 kPa, PH2O=1.4013 kPa) are shown by the dotted line. Open symbol shows the value for eggs raised in shallow pure water(ψπ=0 kPa). No statistical significance between means was observed. Values are means ±1 s.e.m. N values for each treatment are as listed for the variable `Total length' in Table 3.

                    Some Recent Findings

                    • Anosmin-1 is essential for neural crest and cranial placodes formation in XenopusΑ] "During embryogenesis vertebrates develop a complex craniofacial skeleton associated with sensory organs. These structures are primarily derived from two embryonic cell populations the neural crest and cranial placodes, respectively. . Anos1 was identified as a target of Pax3 and Zic1, two transcription factors necessary and sufficient to generate neural crest and cranial placodes. Anos1 is expressed in cranial neural crest progenitors at early neurula stage and in cranial placode derivatives later in development. We show that Anos1 function is required for neural crest and sensory organs development in Xenopus, consistent with the defects observed in Kallmann syndrome patients carrying a mutation in ANOS1."
                    • EphA7 regulates claudin6 and pronephros development in XenopusΒ] "Here we studied the roles of the Eph receptor EphA7 and its soluble form in Xenopus pronephros development. EphA7 is specifically expressed in pronephric tubules at tadpole stages and knockdown of EphA7 by a translation blocking morpholino led to defects in tubule cell differentiation and morphogenesis. A soluble form of EphA7 (sEphA7) was also identified. . Our work suggests a role of EphA7 in the regulation of cell adhesion during pronephros development, whereas sEphA7 works as an antagonist."
                    • N1-Src kinase is required for primary neurogenesis in Xenopus tropicalisΓ] "The presence of the neuronal-specific N1-Src splice variant of the C-Src tyrosine kinase is conserved through vertebrate evolution, suggesting an important role in complex nervous systems. The Src family of non-receptor tyrosine kinases act in signalling pathways that regulate cell migration, cell adhesion and proliferation. Srcs are also enriched in the brain where they play key roles in neuronal development and neurotransmission. Vertebrates have evolved a neuron-specific splice variant of C-Src, N1-Src, which differs from C-Src by just five or six amino acids. N1-Src is poorly understood and its high similarity to C-Src has made it difficult to delineate its function. Using antisense knockdown of the n1-src microexon, we have studied neuronal development in the Xenopus embryo in the absence of n1-src, whilst preserving c-src Loss of n1-src causes a striking absence of primary neurogenesis, implicating n1-src in the specification of neurons early in neural development." Neural System Development

                    This table allows an automated computer search of the external PubMed database using the listed "Search term" text link.

                    • This search now requires a manual link as the original PubMed extension has been disabled.
                    • The displayed list of references do not reflect any editorial selection of material based on content or relevance.
                    • References also appear on this list based upon the date of the actual page viewing.

                    References listed on the rest of the content page and the associated discussion page (listed under the publication year sub-headings) do include some editorial selection based upon both relevance and availability.

                    <pubmed limit=5>Frog Embryology</pubmed>

                    • Endocrine disruption by environmental gestagens in amphibians - A short review supported by new in vitro data using gonads of Xenopus laevisΔ] "Endocrine disruption caused by various anthropogenic compounds is of persisting concern, especially for aquatic wildlife, because surface waters are the main sink of these so-called endocrine disruptors (ED). In the past, research focused on (anti)estrogenic, (anti)androgenic, and (anti)thyroidal substances, affecting primarily reproduction and development in vertebrates however, other endocrine systems might be also targeted by ED. Environmental gestagens, including natural progestogens (e.g. progesterone (P4)) and synthetic progestins used for contraception, are supposed to affect vertebrate reproduction via progesterone receptors. In the present paper, we review the current knowledge about gestagenic effects in amphibians, focussing on reproduction and the thyroid system. In addition, we support the literature data with results of recent in vitro experiments, demonstrating direct impacts of the gestagens levonorgestrel (LNG) and P4 on sexually differentiated gonads of larval Xenopus laevis. The results showed a higher susceptibility of female over male gonads to gestagenic ED. Only in female gonads LNG, but not P4, had direct inhibitory effects on gene expression of steroidogenic acute regulatory protein and P450 side chain cleavage enzyme, whereas aromatase expression decreased in reaction to both gestagens. Surprisingly, beyond the expected ED effects of gestagens on reproductive physiology in amphibians, LNG drastically disrupted the thyroid system, which resembles direct effects on thyroid glands and pituitary along the pituitary-thyroid axis disturbing metamorphic development. In amphibians, environmental gestagens not only affect the reproductive system but at least LNG can impact also development by disruption of the thyroid system." Gonad Development | Thyroid Development
                    • Review - Xenopus Limb bud morphogenesisΕ] "Xenopus laevis, the South African clawed frog, is a well-established model organism for the study of developmental biology and regeneration due to its many advantages for both classical and molecular studies of patterning and morphogenesis. While contemporary studies of limb development tend to focus on models developed from the study of chicken and mouse embryos, there are also many classical studies of limb development in frogs. These include both fate and specification maps, that, due to their age, are perhaps not as widely known or cited as they should be. This has led to some inevitable misinterpretations- for example, it is often said that Xenopus limb buds have no apical ectodermal ridge, a morphological signalling centre located at the distal dorsal/ventral epithelial boundary and known to regulate limb bud outgrowth. These studies are valuable both from an evolutionary perspective, because amphibians diverged early from the amniote lineage, and from a developmental perspective, as amphibian limbs are capable of regeneration. Here, we describe Xenopus limb morphogenesis with reference to both classical and molecular studies, to create a clearer picture of what we know, and what is still mysterious, about this process." Limb Development
                    • Variation in the schedules of somite and neural development in frogsΖ] "The timing of notochord, somite, and neural development was analyzed in the embryos of six different frog species, which have been divided into two groups, according to their developmental speed. Rapid developing species investigated were Xenopus laevis (Pipidae), Engystomops coloradorum, and Engystomops randi (Leiuperidae). The slow developers were Epipedobates machalilla and Epipedobates tricolor (Dendrobatidae) and Gastrotheca riobambae (Hemiphractidae). . We propose that these changes are achieved through differential timing of developmental modules that begin with the elongation of the notochord during gastrulation in the rapidly developing species. The differences might be related to the necessity of developing a free-living tadpole quickly in rapid developers."
                    • Unfertilized frog eggs die by apoptosis following meiotic exitΗ] "Here, we report that the vast majority of naturally laid unfertilized eggs of the African clawed frog Xenopus laevis spontaneously exit metaphase arrest under various environmental conditions and degrade by a well-defined apoptotic process within 48 hours after ovulation. The main features of this process include cytochrome c release, caspase activation, ATP depletion, increase of ADP/ATP ratio, apoptotic nuclear morphology, progressive intracellular acidification, and egg swelling. Meiotic exit seems to be a prerequisite for execution of the apoptotic program."
                    • Dorsal-Ventral patterning: crescent is a dorsally secreted Frizzled-related protein that competitively inhibits Tolloid proteases⎖] "In Xenopus, dorsal-ventral (D-V) patterning can self-regulate after embryo bisection. This is mediated by an extracellular network of proteins secreted by the dorsal and ventral centers of the gastrula. . In sum, Crescent is a new component of the D-V pathway, which functions as the dorsal counterpart of Sizzled, through the regulation of chordinases of the Tolloid family."
                    • Aging of Xenopus tropicalis eggs leads to deadenylation of a specific set of maternal mRNAs and loss of developmental potential⎗]
                    • Repression of zygotic gene expression in the Xenopus germline.⎘] "Primordial germ cells (PGCs) in Xenopus are specified through the inheritance of germ plasm. During gastrulation, PGCs remain totipotent while surrounding cells in the vegetal mass become committed to endoderm through the action of the vegetal localized maternal transcription factor VegT. We find that although PGCs contain maternal VegT RNA, they do not express its downstream targets at the mid-blastula transition (MBT)."

                    Genetics & Embryology Microscope Slides

                    For those studying embryology, you’ll want to go with Carolina microscope slides.  Choose from our multitude of slide sets designed to help educate those studying biology.  From sets that show the development of chicks, frogs and other organisms, you’ll be able to demonstrate the processes to your students using these embryology slides! 

                    We sell whole mount embryology slides as well.  No matter what experience level you’re at, we have a prepared set for you.  Carolina microscope slides are made with teaching and education in mind so you know you’re getting the best quality samples.  Browse from our wide selection below to take advantage of our great prices!

                    Get Teacher Tips and Exclusive Offers

                    Sign up to receive useful teacher tips and exclusive discounts, starting with $25 off your next order.

                    Watch the video: Frog Embryology (January 2022).