Answer of Question of Descriptive Embryology

Answer of Question of Descriptive Embryology


Developmental biology is concerned with the emergence of order and complexity ring the development of a new individual from a fertilized egg, and with the control of

this process. The early preformation concept of development gave way in the eighteenth – ntury to the theory of epigenesis, which holds that development is the progressive

pearance of new structures that arise as the products of antecedent development. ertilization of an egg by a sperm restores the diploid number of chromosomes and tivates the egg for development. Both sperm and egg have evolved devices to promote ficient fertilization. The sperm is a highly condensed haploid nucleus provided with a comotory flagellum. Many eggs release chemical sperm attractants, most have surface ceptors that recognize and bind only with sperm. of their own species, and all have eveloped devices to prevent polyspermy.

During cleavage the embryo divides rapidly and usually synchronously, producing a ulticellular blastula. Cleavage is greatly influenced by the quantity and distribution of olk in the egg. Eggs with little yolk, such as those of most marine invertebrates, divide ompletely (holoblastic) and usually have indirect development with a larval stage terposed between the embryo and adult. Eggs having an abundance of yolk, such as ose of birds, reptiles, and m’ost arthropods divide only partially (meroblastic) and birds

  • nd reptiles have no larval stage.

Based on several development characteristics, bilateral metazoan animals are ivided into two great lineages. The protostomia are characterized by spiral cleavage, osaic cleavage, and the mouth forming at or near the embryonic blastopore. The euterostomia are characterized by radial cleavage, regulative cleavage, and the mouth rming secondarily and not from the blastopore.

At gastrulation, cells on the embryo’s surface move inward to form the germ layers endoderm, ectodern, mesoderm) and the embryonic body plan. Like cleavage, astrulation is much influenced by the quantity of yolk.

Despite the different developmental fates of embryonic cells, every cell contains a omplete genome and thus the same nuclear information. Early development is governed

  • y the products of the maternal genome because the cortex of the egg contains ytoplasmic determinants, deposited during oogenesis, that guide development through leavage. With the approach of gastrulation, control gradually shifts from maternal to mbryonic as the embryo’s own nuclear genes begin transcribing mRNA.

The harmonious differentiation of tissues depends in large part on induction, the ability of one tissue to produce a specific developmental response in another. In vertebrates, cell movements that establish the body plan are coordinated by a primary

organizer; in amphibians the primary organizer is centered in the dorsal lip of the blastopore. Induction guides a sequence of local events, with each step serving as a preliminary for the next step in a developmental hierarchy.

During development, certain parts of each cell’s genome are expressed while the remainder are switched off. Genes expressed early in development produce proteins that regulate the expression of subordinate genes in the developmental hierarchy. One group of control genes, called homeobox genes, encodes regulatory proteins that contain highly conserved DNA binding regions called homeodomains. Homeobox genes control subdivision of the embryo into different developmental fates along the anterior posterior axis.

The postgastrula stage of vertebrate development represents a remarkable conservation of morphology when jawed vertebrates from fish to humans exhibit features common to all. As development proceeds, species- specific characteristics are formed.

Amniotes are terrestrial vertebrates that develop extraembryonic membranes during embryonic life. The four membranes are amnion, allantois, chorion, and yolk sac, each serving. a specific life-support function for the embryo that develops within a self contained egg (as in birds and reptiles) or within the maternal uterus (mammals).

Mammalian embryos are nourished by the placenta, a complex fetal- maternal structure that develops in the uterine wall. During pregnancy the placenta becomes an independent nutritive, endocrine, and regulatory organ for the embryo.

The germ layers tormed at gastrulation differentiate into tissues and organs. The ectoderm gives rise to the skin and nervous system; the endoderm gives rise to the alimentary canal, pharynx, lungs and certain glands; and the mesoderm forms the muscular, skeletal, c.rculatory, and excretory systems.

Answers to the Questions

0.1. What is embryology?

Ans. Embryology (Gr. embryo, to be full + logg, iscourse) is the biological science

which deals with the study of ontogenetic development, viz, embryogenetic and blastogenetic development of an organism. OR. The study of animal development from the fertilized egg to the formation of all major organ system.

Q.2.  How do descriptive and experimental embryology differ?

Ans. Descriptive embroyology: For centuries observation and description of different embryonic stages of the ontogenetic development of a species have remained the chief concern of early embryologists. This type of embryological knowledge constitutes the descriptive embryology.

Experimental embryology: The field of embryology which attempts to understand those fundamental developmental mechanisms which are involved in different animals is called, experimental embryology. This is youngest and most vigorously prosecuted field of embryology and it has provided clear cut understandings regarding concept of gradients, fertilization, cleavage, gastrulation, embryonic induction determination and differentiation.

Q.3. Describe the ideas of preformation and epigenesis.

Ans. As recently as the eighteenth century the prevailing view was that the egg or sperm contains a preformed, miniature embryo that simply grows during its development This idea of preformation came to include the notion that the embryo must contain all its descendants: a series of successively smaller embryos within embryos, like Russian nesting dolls.

The competing theory of embryology was an idea called epigenesis, originally proposed 2000 years earlier by Aristotle that the form of an embryo emerges gradually from a relatively formless egg. As microscopy improved during the nineteenth century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among the embryologists.

Q.4. Describe the experiments of Roux and Driesch.

Ans.  In separate experiments, Wilhelm Roux (1888) and Hans Driesch (1892) set out to determine whether epigenesis or preformation was correct. Both allowed a fertilized frog, toad, salamanders), killed one of the two cells with a hot needle. Driesch using echinoderm embryos (sea stars, urchins, sea cucumbers), completely separated the divided cells. An entire animal developing from a single cell would support epigenesis. A portion of the animal. developing would favor preformation. Interestingly, Roux described the formation of a half embryo that he called a “Hemiembryo”, and Driesch found that each cell retained the potential to develop into an entire organism. Biologists now know that Driesch was the most correct of the two and that the killed cell, still attached to Roux’s developing amphibian embryo, and • probably altered the – development of the untreated cell Fig. 8.1.


Q.5. What is fertilization? What is the role of following in fertilization (a) acrosome (b) egg binding proteins (c) egg activation (d) membrane and cortical events.

Ans: Fertilization: Fertilization is a complex process which involves the fusion of a male gamete (sperm) and a female gamete (ovum). Fundamentally, fertilization has dual functions

1)     To cause the egg to start developing, and

2)     to inject a male haploid nucleus into the egg cytoplasm. i.e., done by:

i)      the activation of egg cortex to form fertilization membrane outside the egg
plasma membrane,

ii)     the activation of egg cytoplasm (more specifically the endoplasm) for the start of various metabolic reactions, and

iii)    the stimulation of mitosis for the cleavage by the contribution of sperm’s centriole to the egg.

(a) The Acrosomal Reaction:

For fertilization to occur, a sperm must penetrate the gel coat of an egg, which consists of protein, or protein and polysaccharide (mucopolysaccharide). When a sperm cell is exposed to molecules from the slowly dissolving jelly coat that surrounds an egg, a vesicle at the tip of the sperm called the acrosome discharges its contents by exocytosis. This acrosomal reactin releases hydrolytic enzymes (lysins) that enable an elongating structure called the


acrosomal process to penetrate the jelly coat of the egg. The tip of the acrosomal process is coated with a protein that adheres to specific receptor molecules located on the vitelline layer just external to the plasma membrane of the egg. The acrosomal reaction leads to the fusion of sperm and egg plasma membranes and the entry of a single sperm nulceus into the cytoplasm of the egg. The acrosome of some species reorganizes into an acrosomal process after releasing lysins. Fig. 8.2.

(b) Egg binding proteins:

Just outside the egg plasma membrae is the vitelline layer (or zona pellucida). Egg binding proteins  (bindins) on the surface of the acrosomal process bind to sperm attachment molecules on the vitelline layer of the egg plasma Acrosomal and egg plasma membranes then fuse. Other parts of the sperm (ag-  the mitochondria, centrioles, and. flagellum) may or may not enter the egg, depending on the species involved. Fig. 8.3.


  • (c) Egg Activation:

Immediately after the apical tip of the acrosomal tubule of a spermatozoon ‘ touches the surface of the egg plasma membrane, fusion of both membranes (i.e., plasma membranes of sperm and egg) over this limited area of contact, takes place and a single continuous mosaic membrane is formed. Thus, the plasma membrane of both gametes (sperm and ovum) becomes in continuation and forms a single cell, called zygote. At this very time, certain very improtant biochemical changes occur.

(d) Membrane and Cortical Events:

Some of the earliest changes in the zygote occur at the plasma membrane and in the outer region of the cell cytoplasm (called cortex). These early changes ensure fertilization by only a single sperm. Single sperm fertilization is important because multiple fertilization usually results in genetic imbalances and a non viable embryo. After contact by sperm, microvilli from the plasma membrane of the ovum wrap around a single sperm. Contraction of microfilaments in the egg’s cytoplasm then draws the sperm into the egg. A second series of events defends against multiple fertilization. Within milliseconds of penetration by a sperm, ionic changes make the plasma membrane unresponsive to other sperm and initiate the formation of a protective envelope around the egg, called the fertilization membrane. The fertilization membrane forms as granules in the cortex discharge into the region between the egg plasma membrane and the vitelline layer. The cortical granules release enzymes that loosen the vitelline layer’s contact with the plasma membrane. The granules allow water to enter the space between the vitelline layer and the egg plasma membrane, causing the vitelline layer to lift off the egg. Proteins of the cortical granules thicken and strengthen the vitelline layer. All of these reactions are completed in ito 2 minutes following fertilization. Other important changes occur in the egg cortex. After sperm penetration, the cortical layer thickens, and rotational and sliding movements of the outer egg cytoplasm begin. In amphibians, these cortical changes result in the formation of a gray crescent on the egg, opposite the point of sperm penetration. The gray crescent has an important influence on later development.

Q.6.  How do the animal and vegetal poles of an egg differ?

Ans. The eggs of most animals have a definite polarity, and the planes of division during cleavage follow a specific pattern relative to poles of the zygote. The polarity is defined by concentration gradients of cytoplasmic components in the egg, such as mRNA, proteins, and yolk. In many frogs and other animals, the concentration gradient of yolk is a key factor in determining polarity and influencing the pattern of cleavage. Yolk is most concentrated at one pole of the egg, called the vegetal pole, while the opposite pole, the animal pole, has the lowest concentration of yolk,, more mitochondria, and more ribosomes and is more metabolically active. The animal pole is also the site where the polar bodies of meiosis are budded from the cell, and in most animals, it marks the point where the most anterior part of the embryo will form.

Q.7.  What is the role of quantity and distribution of yolk?

Ans. Egg sizes, cleavage patterns, and the length of embryonic periods of animal species are related to differences in the quantity and distribution of yolk in an egg. Yolk, a mixture of proteins, lipids, and glycogen, is the food reserve for the developing embryo. Animals with relatively small amounts of yolk — for example, echinoderms (sea stars and their relatives) and amphibians (frogs and their relatives) often have larval stages that begin to feed after a brief period of embryological development during which yolk is entirelly absorbed. Some animals with longer periods of embryological development (reptiles and birds) provide embryos with larger quantities of yolk. Other animals with long periods of embryological development (eutherian, or placental, mammals and some sharks) provide nourishment to the embryos through a placenta or some other modification of the female reproductive tract.

Q.8. What is cleavage? How does the amount of yolk affect cleavage? How do holoblasitc and meroblastic cleavage differ?

Ans. During cleavage the embryo divides repeatedly to convert the large, unwiedly cytoplasmic mass into a large cluster of small, maneuverable cells (called blastomeres). There is no growth during this period, only subdivision of mass, which continues until normal somatic cell size is attained. cleavage is greatly The pattern of affected by quantity and i)       genes 7 controlling the symmetry of cleavage. How amount and distribution of yolk ROTATIONAL HOLOBLASTIC affect cleavage:


  1. Eggs with very little yolk that is evenly E distributed in the egg are called isolecithal. In such eggs, cleavage is holoblastic (Gr. holo, whole + blastos, germ), meaning that the cleavage furrow extends completely through the egg. Isolecithal eggs are found in a great diversity of animals, including echinoderms, tunicates, cephalochordates, nemerteans, most molluscs, as well as marsupial and placental mammals (including humans).

2. Amphibian eggs are called mesolecithal (Gr. meses, middle, + ;lekithos, yolk) because they have a mederate amount of yolk, and they also cleave holoblastically, but cleavage is substantially retarded in the yolk — rich vegetal pole. Each cleavage furrow begins at the animal pole and extends towards the vegetal pole.

In axolotl salamanders, the cleavage furrow moves through the animal

hemisphere at a rate of about lmm / min; it slows down to a rate of about 0.02 mm/min as it moves through the vegetal hemisphere. As a result, the second cleavage division begins at the animal pole while the first cleavage furrow is still slicing through the vegetal hemisphere. As cleavage progresses, the animal region becomes packed with numerous small cells, while the vegetal region contains relatively few, large, yolk – filled cells.

  1. Eggs of birds, reptiles, most fishes, a few amphibians, cephalopod molluscs, and monotreme mammals are called telolecithal, because they contain an abundance of yolk that is densely concentrated at the vegetal pole of the egg. The actively dividing cytoplasm is confined to a narrow disc shaped mass lying on top of the yolk. Cleavage is partial, or meroblastic (Gr. meros, part, + blastos, germ), because the cleavage furrows cannot cut through the heavy yolk concentration, but instead stop at the border between the cytoplasm and yolk below.
  2. Centrolecithal eggs, ty0ical of insects and many other arthropods, also exhibit meroblastic cleavage. These eggs have a large mass of centrally located yolk and cytoplasmic cleavage is limited to a surface layer of yolk – free cytoplasm while the yolk rich inner cytoplasm remains uncleaved.


Thus, yolk is an important factor to cleavage. In eggs with relatively little yolk, cleavage furrows can cut through the cytoplasm relatively easily and cleavages are therefore holoblastic. Once yolk becomes highly concentrated within portions of the egg, cleavage furrows cannot penetrate the yolk and cytoplasmic cleavage is limited to relatively yolk – free areas, yielding a meroblastic type of cleavage.

Q.9.  What tissue Is derived from each of the following germ layers: (a) ectoderm, (b) endoderm, and (c) mesoderm?

Ans. Tissues and organs of animals arise from layers, or blocks, of embryonic cells called primary grerm layers. The outer layer ectoderm (Gr. ektos, outside + derm, skin) gives rise to the epithelium of the body surface and to the nervous system. The inner layer that forms the archenteron is endoderm (Gr. endo, within) that forms the epithelial linig of the digestive tube. The outer pocketing of the archenteron is the origin of mesoderm (Gr. meso, in the middle). It gives rise to tissues between ectoderm and endoderm. Undifferentiated mesoderm (called mesenchyme) develops into muscles, blood and blood vessels, skeletal elements, and other connective tissues.


Q.10. What changes in an echinoderm egg lead to the formation of blastula?

Ans. The eggs of echinoderms have relatively little yolk that is evenly distributed throughout the egg Cleavages are holoblasfic that result in similarly sized blastomeres. In just a few hours, a solid ball of small cells, called the morula (L. morum, mulberry) is produced. As cell division continues, cells pull away from the interior of the embryo. A fluid — filled cavity, the blastocoel, forms, and the cells form a single layer around the cavity. The embryo is now a hollow sphere called a blastula. In sea urchins, development through the blastula stage takes place within the fertilization membrane. When the cells of the blastula develop cilia, the blastula breaks out of the fertilization membrane and begins to swim. Late in the blastula stage, groups of cells break free of the animal end of the embryo and position themselves within the blastocoel. These cells. called primary mesenchyme, will form skeletal elements (called spicules) of the embryo.


Q.11. Describe the gastrulation and morphogenesis in echinoderms. Ans. Gastrulation

Ans. The first sign of gastrulation in echinoderms is the invagination of cells at a point in the vegetal half of the embryo. The point of invagination is the blastopore, which will eventually form the anal opening of the larva. During invagination, an embryonic gut, the archenteron (Gr. archeo, ancient enteron, gut), elongates and reduces the size of the blastocoel. During gastrulation, the embryo also begins to lengthen and assumes a pyramidal shape. Although adult echinoderms do not have head and tail ends, larva has a preferred direction of movement. The end of an animal that meets the environment during locomotion is called the anterior end and is where the head of most animals is located. The opposite end is the posterior end. The shape changes that Occur during gastrulation establish the anteroposterior axis of the embryo. Fig. 8.8.


Finally, a body cavity, or coelom, forms from outpockets of the archenteron, and the gut breake through the anterior body wall. The opening thus produced is the mouth. The cell movements that begin in gastrulation  result from groups oft cells changing their shapes simultaneously. Contractile m icrofilaments mediate these shape



  1. These
    precise, coordinated changes transform a single — layered sphere of cells. The progressive development of an animal’s form that begins in gastrulation is morphogensis. In the sea urchin, these changes produce a pluteus larva that swims freely in the sea and feeds on smaller plants and animals.

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Figure 8.8

Sea urchin gastrulation. 1 Formed by cleavage the blastula consists of a single layer of ciliated cells surrounding vegetal plate at the vegetal pole. Mesenchyme cells (future mesoderm) detach from the plate and migrate into the blastocoel. 2 The vegetal plate in this early gastrula invaginates (buckles inward). Mesenchyme cells begin to form extensions (filopoodia). 3 Endoderm cells from the archen-teron (future digestive tube). Mesenchyme cells form filopodial connections between the tip of the archenteron and the ectoderm cells of the blastocoel wall (inset LM) 4 Contraction of the filopodia in a late gastrula drage the archenteron the rest of the way across the blastocoel where the endoderm of the archenteron will fuse with ectoderm of the blastocoel wall 5 Gastrulation is compete. The gastrula has a functional digestive tube formed from the endoderm of the embryo’s ciliated skin. Some of the mesenchyme cells of the mesoderm have secreted mineral that will form a simple interanl skeleton.

Q.12. Describe the chordate body plan Ans. Chordate body plan

Ans. Vertebrates are members of the phylum Chordata, and certain structures characterize all chordates. The endpoint of the study of vertebrate embryology is the point at which most of these characteristic structures have formed.

1. The chordate nervous system develops from ectoderm, and is dorsal and tubular. The first evidence of a developing nervous system is the formation of the neural tube. Nervous tissue proliferates anteriorly into a brain.


  1. The notochord is the primary axial structure in all chordate embryos, as well as many adults. It is flexible, yet supportive, and lies just beneath the neural tube. The notochord is mesodermal in origin and consists of vacuolated cells packed into a connective tissue sheath.
  2. In addition to the notochord and the dorsal tubular nerve cord, ‘all chordates possess pharyngeal slits or psuches and a postanal some point in their life history. Fig. 8.9.

Q.13. Describe different events of developmental process in Amphibians. Ans. Amphibian Embryology

Ans. Most amphibians lay eggs in watery environments and the eggs are fertilized as the female releases them. Frog eggs have a pigmented animal pole. Because the vegetal pole is heavily laden with yolk, the eggs rotate in their jelly coats so that the less dense, darkly pigmented animal pole is oriented up. This rather simple series of events has interesting adaptive significance. Fig. 8.4c. Amphibian eggs usually develop with little care or protection from the parents. The pigmentation helps camouflage developing embryos from predators by the following ways:

  1. When viewed from below, the light color of the vegetal and of floating eggs blends with the sky above.
  2. When viewed from above, the dark, color of the animal end blends with the bottom of the pond, lake, or stream.
  3. The dark pigment of the animal pole also absorbs heat from the sun, and the warming may promote development.


The first and second cleavages of the amphibian embryo, are longitudinal and at right angles to each other. It begins at the animal pole and divides the gray crescent in half. Because of the large amount of yolk in the vegetal end of the egg, cleavages are slower there than in the animal end. The third cleavages horizontal. Latei cleavages are irregular. The amphibian morula, thereforP consists of many small cells at thp animal end of the embryo and fewer, larger cells at the veguiai end of the embryo.


The cells of the blastula that will grouped on the surface of the blastula. During gastrulation, some of these cells move into the interior of the embryo. The first sign that gastrulation is beginning is the formation of a groove between the gray crescent and the vegetal region of the embryo. This groove is slitlike blastopore. The animal pole margin of the blastopore is the dorsal lip of the blastopore. Cells at the bottom of the groove move to the interior of the embryo, and the groove spreads transversely. Superficial cells begin to roll over the dorsel lip of the blastopore in a process called involution (to curl inward). Cells spread from the animal pole toward the blastopore and replace those moving into the interior of the embryo. In the process, the ends of the slitlike blastopore continue to spread transversely and downward toward the vegetal pole until one end of the slit meets and joins the


Figure 8.10

Gastrulation In a frog embryo. (1) The blastocoel of the frog blastula is off-center and surrounded by a wall that is more than once cell thick. At this stage, the colors indicare regions of the blastula that will become the embryos three germ layers. (2) Gastrulation begins when a small tuck, the dorsal lip of the blastopore, appears on one side of the blastula. The tuck is formed by cells burrowing inward from the surface. Additional cells that will become endoderm mesoderm then roll inward over the dorsal lip (involution) and move away from the blastopore into the interior of the gastrula. Meanwhile, cells of the animal pole, which will form ectoderm, spread over the embryo’s outer surface.(3) Externally, the lip of the blastopore starts becoming cirular. Intennally, the three germs layers start, forming as cells continue migrating inward. The advancing endoderm, mesoderm, and the archenteron, lined by endoderm, are filling the space occupied by the blastocoel. (4) Late in gastrulation. the circular blastopore surrounds a plug of yolk cells (the yolk plug) and the three germ layers are in place, ready for organogenesis.

opposite end of the slit. A ringlike blastopore now surrounds the protruding yolk — filled cells near, the vegetal end of the embryo. These protruding celis are callea the yolk plug. Evetually, the lips of the blastopore contract to competely enclose the yolk. The blasotpore is said to have “closed”. Fig. 8.10.

During the closing of the blasotpore, two other movements occur:

  1. The spreading of cells from the animal pole toward the dorsal lip of the balstopore and the rolling of cells into the blastopore form the archenteron. As these mesodermal and endodermal cells roll into the interior of the embryo, the archenteron becomes larger, and the blastocoel becomes smaller.
  2. Gastrulation results in a spreading and thinning of endodermal cells toward the blastopore. In addition, ectoderm spreads over the entire embryo, a


the neural tube Tissue at the meeting margins of the tube separate from the tube as the neural crest, a source of migrating cells that eventually form many structures , including bones and muscles of the skull, skin pigments cells, adrenal medulla glands, and peripheral gangila of the nervous system. (c) in this cross section, an embryo with a completed neural tube has somites flanking the notochord. Formed from mesoderm, the somites will give rise to segmental structures such as vertebrae and serialy arranged skeletal muscles. The lateral mesoderm has begun to separate into the two tissue layers that line the coelom. In the scanning electron micrograph of a whole embryo at the tail-bud stage (side view), part of the ectoderm has been removed to reveal the somiles.

Mesoderm Formation

Some of the last cells to roll over the dorsal lip into the blastopore are presumptive notochord and presumptive mesoderm. Initially, these cells makeup the dorsal lining of the archenteron near the blastopore. Later, they detach from the endoderm and move to a position between the endoderm and ectoderm in the region of the dorsal lip of the blastopore. This mesoderm, called chordamesoderm, differentiates into notochord.

Later to the notochord, mesoderm spreads and thickens along the sides of the embryo. These thickenings, called somites are visible externally as a row of bumps on either side of the embryo. As mesoderm continues to spread ventrally, it splits to form the body cavity (coelom) and the mesodermal lining of the body wall and gut. Fig. 8.10.

Neural Tube formation

During late gastrulation, external changes along the upper surface of the embryo begin to form the neural tube — a process called neurulation. After gastrulation is complete, an oval — shaped area on the dorsal side (the future backside) of the embryo marks the presumptive neural tube. This region is the neural plate. Microfilaments in neural plate cells flatten and thicken the neural plate. The edges of the neural plate roll up and over the midline of the neural plate. These longitudinal ridges, called neural folds, meet dorsally to form the neural tube. The portion of the neural tube that will become the brain is the last to close. Fig.8.11.


With further development of the mesoderm, the amphibian embryo gradually takes on the forms of a tadpole larva. Yolk in cells lining the floor of the gut is gradually depleted, and the larva begins feeding on algae and other plant material.

Q.14. Describe the different events of embryonic development in birds.

Ans. Avian embryology:

The yellow portion of the chicken egg is the single cell produced in the chicken ovary. This egg is released into the oviduct, where fertilization may occur. Following fertilization, membranes and fluids collect around•the egg. A vitelline membrane covers the surface of the true egg. The “white” consists of water and a protein called. albumen. This watery environment protects the egg from mechanical damage and drying. Albumen is a source of nutrients (in addition to the yolk of the egg) and is eventually consumed during development. Two denser strands of albumen (called chalazae) attach to the inside of the shell and to the egg, and suspend the egg in the center of the watery albumen. The shell is made of calcium carbonate impregnated with protein. Thousands of tiny pores (40 to 50 nm in diameter) in the shell permit gas exchange between the embryo and the outside. On the inside of the shell are two shell membranes. An air pocket forms between these membranes at the rounded end of the shell. The air pocket enlarges during development, as air moves through pores in the shell to replace water loss. As hatching approaches, the chick penetrates the air pocket with its beak, the lungs inflate, and chick begins to breathe from the air sac, while still exchanging gases across vascular extraembryonic membranes. Fig. 8.4d.


Cleavage of the chicken egg is meroblastic. A small disk of approximately sixty thousand cells at the animal end of the egg develops, that is the blastoderm. The blastoderm is raised off the yolk, leaving a fluid filled space analogous to the blastocoel of the amphibian blastula. Proliferation and movement of blastoderm cells sort the cells into two layers.

  1. The epiblast (Gr. epi , upon + blast, sprout) is the outer layer of cells, and
  2. The hypoblast (Gr. hypo, below) is the inner layer.


The movements of blastoderm cells are the beginning of gastrulation. The female reproductive tract releases the egg at about this time.


A medial, linear invagination, called the primitive streak, gradually extends anteriorly. A depression, called Henson’s node, forms at the anterior margin of the primitive streak and marks the beginning of an inward migration of epiblast cells, compareable to involution of the amphibian gastrula. The primitive streak is, therefore, analogous to the drosal lip of the blastopore. This migration occurs during a dramatic posterior movement of Henson’s node. Migrating cells form mesoderm, what is left of the epiblast on the surface of the embryo is the ectoderm, and the hypoblast forms the endodermal lining of the gut tract. The three germ layers are now arranged above the surface of the yolk.

Neural tube formation:

Following gastrulation, notochordal cells separate from the overlying neural ectoderm, and the neural tube forms. In addition, mesoderm, which originally formed as solid blocks of cells, organizes into somites and splits to form the The embryo lifts off the yolk when the margins of the embryo grow downward and meet below the embryo. A connection between the embryo and the yolk is retained and is called the yolk stalk. Blood vessels develop in the yolk stalk and carry nutrients from the yolk to the embryo. Fig. 8.12.

Q. 15. What are the extra embryonic membranes in terrestrial animals?

Ans. The development of extraembryonic membranes:

Extraembryonic membranes of amniotes include the yolk sac, the amnion, the chorion, and the allantois. Reptiles and birds have a large quantity of yolk that becomes enclosed by a yolk sac. The yolk sac develops from a proliferation of the endoderm and mesoderm around the yolk. The yolk sac is highly vascular and distributes untrients to the developing embryo.

Amnion and chorion:

Following the neural tube stage, the ectoderm and mesoderm on both sides of the embryo lift off the yolk and grow


Figure 8.13

Early development of a human embryo and Its extraembryonlc membr­anes. This series of drawings illustrates four stages in transverse sect­ion. 1 Cleavage produces a blastocyst, consisting of a trophoblast surrounding a blastocoel and an inner cell mass. The blastocyst imp­lants in the uterine lining 2 Coincident with implantation the inner cell mass forms an epiblast cell layer which will develop into [tie three germ layers of the embryo and a hypoblast which will form the yolk sac. 3 By this stage the trophoblast has begun to form the chorion and continues to expand into the endometrium. The epiblast has begun to form the amnion, surrounding a fluid-filled cavity. Mesodermal cells that will become part of the placenta are also derived from the epiblast. 4 Gast­rulation by the inward movement of epiblast cells has produced a three layered embryo surrounded by proliferating extraembryonic mesoderm.

dorsally over the embryo. As these membranes meet dorsally, they fuse and form an inner amnion and an outer chorion. The amnion encloses the embryo in a fluid — filled sac. This amniotic cavity protects against shock and drying. The chorion is nearer the shell, becomes highly vascular, and aids in gas exchange. Fig. 8.13.


The immediate breakdown product of proteins is highly toxic ammonia. This ammonia is converted to a less toxic form, uric acid, which is excreted and stored in allantois, a ventral outgrowth of the gut tract. Uric acid is semisolid, and thus, little water is wasted. The allantois gradually enlarges during development to occupy the region between the amnion and chorion. In addition, the allantois becomes highly vascular and functions with the chorion in gas exchange.

Q.16. What structures are derived from embryonic mesoderm? The fate of mesoderm

Ans. Following gastrulation in birds, reptiles, and mammals all three primary germ layers have forrned. Of the three layers, the fate of mesoderm is the most comp

Mesoderm Formation: In amphioxus, pouches of hypoblast material along the dorsal wall of the archenteron push or evaginate, to the right and left of the middorsal line, to give rise to tissue now known as mesoderm. The cavities formed in these pouches develop and remain as permanent cavities and are the first evidence of the coelom (enterocoel). In most vertebrates, however, the mesoderm masses form as solid sheets and later split which is the beginning of the coelom (schizocoel). Fig. 8.6, 8.14.


Notochord: On the dorsal wall of the hypoblast, a mass of cells pinches off along the midline to become the notochord. This lies dorsal to the hypoblast and between the two mesodermal masses. Differentiation of Mesoderm: The mesodermal masses formed on either side of the notochord (between the ectoderm and endoderm) now differentiate further.

The mesoderm lateral to the notochord and well above the coelom becomes the epimere. The mesoderm forming the dorsal wall of the coelom on each side becomes the mesomere. The mesoderm which forms the inner and outer walls of the coelom is known as hypomere. The hypomere on the outer wall of the coelom is called somatic hypomere and that on the inner wall, splanchinc hypomere.

Mesoderm and Its Derivatives

Much of the mesoderm forms structures directly in place, but part of it remains as an undifferentiated, loosely organized tissue known as mesenchyme, which has the ability to migrate to other regions of the body and to differentiate there. Mesenchyme gives rise to the circulatory system and to muscle, bone, and connective tissue. The primary divisions of the mesoderm, as mentioned above, are epimere, mesomere, and hypomere.

1. Epimere

On each side of the body, the epimere segments into a longitudinal row of blocks, the mesodermal somites, each of which further differentiates into three parts i.e., sclerotome, dermatome and myotome. A brief account of all these is given below.

(a)      Sclerotome: The medial portion of the epimere, next to the notochord and neural tube, is known as sclerotome. It gives rise to vertebral structures which surround the notochord and the nerve cord, and to mesenchyme which forms bony and cartilaginous structures elsewhere.

(b)      Dermatome: The lateral portion of the epimere that is in contact with the skin ectoderm is known as dermatome. It forms the dermis, the inner layer of the skin.

(c)      Myotome: The portion of the epimere between the median sclerotome and the lateral dermatome is the myotome. The myotomal segments, which are separated from one another by septa called myocomma or myosepta, give rise to the large skeletal muscle masses that make up such as large part of the vertebrate body.

2. Mesomere

The mesomere also called neurogenic mesoderm develops into the urogenital organs (kidney and gonads) and their ducts. The terminal portions of the ducts are sometimes lined with ectodermal, or occasionally endodermal, epithelium.

3. Hypomere

The hypomere is divided into an inner, splanchnic hypomere and an outer, somatic hypomere. A bride account of these parts is given below:

(a)      Splanchnic Hypomere: The splanchnic hypomere fuses with the endoderm that constitutes the wall of the gut to form splanchnopleure. Splanchnic hypomere is largely mesenchymal in nature and gives rise to such structures as the connective tissue and smooth muscle of the gut, the heart and its related blood vessels, and various mesenteries and ligaments.

(b)      Somatic Hypomere: The somatic hypomere becomes associated with the ectoderm of the body wall to form the somatopleure. Somatic hypomere contributes to such tissue as the pericardium which surrounds the heart, the pleura which covers the lungs and lines the pleural cavity, and the peritoneum.


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