BSC ZOOLOGY DEVELOPMENTAL BIOLOGY NOTES

The Structure of a typical Ovum

Ovum is the female gamete. lt stores food required for the entire process of development in the form of yolk. lt has three important f unctions:

1. lt supplies a haploid set of chromosomes to the future embryo.

2. lt contributes almost all cytoplasm to the zygote.

3. lt supplies food to the developing embryo.

Shape and Size

Typically, the eggs are spherical or ovoid in shape. But in a few animals like insects, the eggs are elongated and cylindrical in nature. Eggs are generally large rthan the sperms and average somatic cells. The size of a mature egg depends on the amount of yolk present in it. The smallest known egg is that of mouse (0.07mm); the birds possess larger eggs. Ostrich lays the largest egg having a diameter of about 85 mm.

The egg is covered externally by a plasma membrane or plasmalemma. Within the plasma membrane is the granularcytoplasm

Organisation of Egg Cytoplasm

The cytoplasm of egg cell is known as ooplasm. lt is granular and contains in addition to the usual cellular organelles certain other inclusions Iike yolk, pigments and cortical granules. The peripheral layer of ooplasm is more viscous and gelatinous. lt is known as the egg cortex which is provided with many microvilli and cortical granules. The microvilli are formed by the outpushings of the plasmalemma and they help in transportation of substances from the outside into the ooplasm during the development of egg. The cortical granules are very small spherical bodies varying in diameter from 0.8 µm to 2µm . They are

rnembrane bound and are formed from golgi complex. They contain homogeneous and granular mucopolysaccharides. Cortical granules are present in the eggs of sea urchins, frogs, fishes, bivalve molluscs, several annelids and certain mammals.

Yolk:Nutritive substances are stored in the cytoplasm of egg in the form of yolk or deutoplasm. This stored food is utilized by the embryo for its early development. The process of formation of yolk is known as vitellogenesis. The yolk is a complex material consisting of proteins, fats, carbohydrates, inorganic salts, vitamins, enzymes, pigments and water. The yolk may be called "protein yolk" when it has more proteins than lipids, or " fatty yolk" when it has more fat contents than the proteins. Most animal eggs contain both kinds of yolk. Since the yolk is heavier ,large quantities of yolk, such as those of the frog and chick, the accumulation of yolk in one region is so marked that they are known as telolecithal eggs. In eggs containing lesser amount of yolk, like those of Amphioxus and man, the york is distributed more uniformry, hence they are known as isolecithatal or  homolecithal.

Pigment granules are present in the cytoplasm of eggs of many species. The granules may be brown, black, red, yellow, green or- grey in colour. As the pigment granules are not common to all eggs, they do not play any significant role in development.

Polarity

The constituents of egg are not uniformly distributed throughout the cytoplasm. These are distributed in such a way that two poles distinct can be identified in the egg. These pores are known as animal pole and vegetal pore. The cytoplasm is concentrated in the upper portion or animal hemisphere and the yolk material is concentrated in the lower portion or vegetal hemisphere. A plane passing through these two poles constitute the polar axis. The nucleus is always located in the polar axis, more or less towards the animal pole. The yolk shows a gradation from the animal pole towards the vegetal pole. There is also a metabolic gradation along the polar axis. Metabolic processes are highest at the animal pole and progressively diminish towards the vegetal pole

Classification of Egg

1. On the Basis of the Amount of yolk

Eggs are grouped into three types on the basis of the amount of yolk present in them.

1. AlecithalEgg;When the egg contains no yolk, it is called alecithal egg.

Eg. The eggs ofeutherian mammals

2. Microlecithal Egg:When the egg contain. small or negligible amount of yolk it is said to be microlecithal.Romer and Balinsky named these eggs as oligolecithal eggs Eg'.Amphioxus, Tunicates

3.Mesolecithal egg: In amphibian, Dipnoi and Petromyzon the amount of yolk present is moderate and is not high Hence these eggs are also named as mesolecithal eggs

4. Macrolecithal or Megalecithal Egg

When the egg contains large amount of yolk it is said to be macrolecithal or megalecithal egg. It is also called Polylecithal egg egg.Eg. boney fishes' amphibians,reptiles and birds.

tnonotentcs, etc

1. On the Basis of the distribution  of yolk

a. Isolecithal or Homolecithal Egg:ln isolecithal eggs, the very little amount of yolk present is uniformly distributed throughout the ooplasm (eg.. echinoderms, Amphioxus, mammals). This condition is usually observed in eggs with very little amount of yolk.

b. Telolecithal Egg:In eggs containing moderate or large quantity of yolk, the distribution of yolk is not uniform. lt is concentrated more towards the vegeial pole. Such a type of egg, in which the yolk is concentrated towards one pole, is called telolecithai egg.

Telolecithal eggs may further classified into three types:

i.Slightly Telolecithal  Thls type  of egg contains only a small quantity of yolk which

is distributed unevenly. The vegetal pole has the highest concentration and the animal pole the lower (e.g. eggs of fishes).

ii.Moderately Telolecithal egg

This type of egg contains a moderate  quanilty  of yolk which is

Distributed  unevenly. Due to high concenteration  of yolk  in the vegetal hemisphere, the nucleus is shifted more towards the animal hemisphere

(eg. amphibian egg).

iii.Extremely Telolecithal Egg

ln this type of egg, due to the heavy deposition of yolk, the entire vegetal hemisphere and a major portion of the animal hemisphere are occupied by yolk. Due to this extremely uneven distribution of yolk, the ooplasm and nucleus are displaced towards the animal pole (e.9. reptilian and avian eggs).

3. Centrolecithal Egg

Egg of many arthropods and some coelenterates are described as centrolecithal. They are relatively large and elongate and have a very great amount of yolk. The nucleus lies at the geometric centre of the yolk mass, surrounded by a small amount of cytoplasm. A thin cytoplasmic layer covers the surface of the yolk. Fine strands of cytoplasm extend from the peripheral layer to the zone occupied by the nucleus.

Mosaic and Regulative Eggs

a.Mosaic Egg: ln certain eggs, every portion is predetermined with respect to its potentialities foi' further development

lf a small portion of such an egg is removed, a defective embryo iS formed, This is because removal of a portion results in a permanent loss from the egg. The remaining portion of the egg cannot make compensatory development to make good the lost part. Such an egg, in which the future developmental potentialities are predetermined in the form of a mosaic, is called mosaic or determinate egg (e.g annelids, Molluscs and ascidians).

b.Regulative Egg

ln vertebrates and most of the invertebrates, the developmental potentialities are not predetermined in the eggs. Removal of a small portion of the egg, or even one or two early blastomeres will not affect the normal development. This type of egg in which the future developmental potentialities are not predetermined is known as regulative or indeterminate egg.

Egg Membranes

The eggs are well protected by egg  membranes' The membranes are produced either by the egg itself or by the follicle cells of the ovary or by the genital ducts (oviduct) of the female, mother.Accordingly, the egg membranes are classified into three types. They

are .1. primary membranes 2. Secondary membranes and 3. Tertiary membrane.

III. primary membranes :The membranes secreted by egg cytoplasm (ooplasm) constitute the primary membrane. They are closely  attached to the surface of the egg. The primary membranes are named differently in the different animals. They are

a. Plasma Membrane

It is the membrane covering the egg immediately It over it. is found in all the.eggs.

in structure, It resembles the plasma membrane of a cell.

b.Vitelline Membrane: it is closely attached to the plasma membrane of egg. Commonly found in  Egg of Amphioxus. molluscs. Echinoderm s, amphibians birds etc It is very thin and and transparent. It is formed of mucopolysacharide and fibrous protein. The space formed between it and the plasma membrane is called perivitelline space filled with a  a

fl uid called perivitelline fluid.

c. Chorion:It is found in the eggs of lower chordates like fishes (styela)It is is a product of surface ooplasm

d.Zona Radiata

Th.e egg of the shark Scyllium canicula, has two primary membranei produced by the surface ooplasm. The outer membrane is the vitetline mbmbrane andthe inner membrane has

a radiating appeafance and hence called zona radinta The eggs-. of teleost fishes are also covered by zona radiate.

e.Zona Pellucida

All mammalian eggs are surrounded by a membrane called zona pellucida is also named as zone radiata.It is so named  because it gives a striated appearance under the microscope'

The striations are due to the presence of microvilli and macrovilli (desmosomes) in this zone. The microvilli are produced  by the surface of the egg and microvilli are produced by

Follicle  cells. They protrude into the zona pellucida

II. Secondary Membranes

The secondary membranes are produced by the follicle cells (cells found around the developing oocytes) ofthe ovary. These membranes are usually tough and jmpermeable' The secondary membranes are as follows

a. Chorion

This is a common outer covering in the eggs of insects, ascidians and cyclostomes (Myxine).It is found outside the vitelline membrane. As the chorion is tough and impermeable

it is provided with one or more openings called micropyles through which the sperms enter the egg

b. Corona Radiata It is found in mammalian eggs. This membrane is formed of a layer of follicle cells. The cells are radially arranged around the zona pellucida

III.Tertiary Membranes

The tertiary membranes are produced by the oviduct.

a. White Albumen

It is found in the egg of hen. It is found outside the vitelline membrane. It is formed of three layers-an inner less dense albumen, a middle dense albumen and an outer less dense albumen. The albumen is formed of water  and protein

b. Shell Membrane:The shell membrane is formed around the albumen in the egg of hen. It is a double membrane. The two membranes adhere closely and are separated by an air space ar the blunt end of the egg. This membrane is formed of keratin-

c. Shell:The shell is the outer covering of land animais eggs. It is formed of calcium carbonate. Lt is white or brown in colour.

It contains as many as 7000 minute pores. These pores are 0.04 to 0.05 mm in diameter. They are filled with  a proteinous substance called collagen.

d. Jelly Coat ,:The amphibian eggs are surrounded by a gelatinous covering called jelly coat

e. Mermaid's Purse

It is the egg case of some cartilaginous fishes. It is a protective hard shell secreted by the shell glands present in the oviduct. The shape of the purse varies from group to group.

Generally ,it is rectangular in shape. The corners of the shell are drawn out into four long twisted elastic filaments which serve to attach the eggs to sea weeds. In dog-fish Chiloscyllium,

development is completed within this purse

Cleavage  and types

The process of cleavage reamains one of the earliest mechanical activity in the conversion of a single celled egg into a multicellular embryo. It is initiated by the sperm during fertilization. However in parthenogenetic eggscleavage can commence without the influence of fertilization.

The process of cleavage or cellulation happens through repeated mitotic divisions. These divisions result in cells called blastomeres. In later stages of development the blastomeres occupy different regions and differentiate into several types of body cells.

The first cleavage of frog’s egg was observed by Swammerdam in 1738. The entire process of cleavage in frog’s egg was studied by Prevost and Dumas in 1824. With the development of microscopes cleavages and further stages were observed in the eggs of sea urchin, star fishes, amphioxus and hen’s eggs.

From all these studies it has become clear that all divisions in cleavage are mitotic. The mitotic process is very rapid. In the eggs of sea urchin division of the blastomeres can be observed every 30 minutes.

 As the cleavage progresses the resultant daughter cells, namely the blastomeres get reduced in size. During cleavage there is no growth in the blastomeres. The total size and volume of the embryo remains the same.

The cleavages result in a compact mass of blastomeres called morula. It gets transformed into blastula. While the wall of the blastula is called the blastoderm, the central cavity is called the blastocoel.

The planes of cleavage

An egg can be divided from different planes during cleavage. Depending on the position of the cleavage furrow the planes of cleavage are named.

1. Meridional plane: The plane of cleavage lies on the animal vegetal axis. It bisects both the poles of the egg. Thus the egg is divided into two equal halves.

2. Vertical plane: The cleavage furrows may lie on either side of the

meridional plane. The furrows pass from animal to vegital pole. The cleaved

cells may be unequal in size.

3. Equatorial plane: This cleavage plane bisects the egg at right angles

to the main axis. It lies on the equatorial plane. It divides the egg into two

halves.

4. Latitudinal plane: It is similar to the equatorial plane, but it lies on either side of the equator. It is also called as transverse or horizontal cleavage.

Influence of yolk on cleavage

Yolk is needed for embryonic development. However the fertilized egg has to undergo all stages of development and result in a suitable ‘young form’ initiating next generation. Somehow with all the influences of yolk the developmental procedures are so adapted and modified that a well formed embryo will result. The initial influence of yolk is felt during the process of cleavage.The amount of the yolk and its distribution affect the process of cleavage.

Accordingly several cleavage patterns have been recognised.

1. Total or holoblastic cleavage - In this type the cleavage furrow bisects

the entire egg. Such a cleavage may be either equal or unequal.

(a) Equal holoblastic cleavage - In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size. Eg: Amphioxus andplacental mammals.

(b) Unequal holoblastic cleavage - In mesolecithal and telolocithal eggs,cleavage leads to the formation of blastomeres of unequal size. Among the blastomeres there are many small sized micromeres and a few large sized macromeres.

2. Meroblastic cleavage - In this type the cleavage furrows are restricted to the active cytoplasm found either in the animal pole (macrolecithal egg) or superficially surrounding the egg (centrolecithal egg). Meroblastic cleavage may be of two types.

(a) Discoidal cleavage - Since the macrolecithal eggs contain plenty of yolk, the cytoplasm is restricted to the narrow region in the animal pole. Hencecleavage furrows can be formed only in the disc-like animal pole region. Sucha cleavage is called discoidal meroblastic cleavage. Eg: birds and reptiles.

(b) Superficial cleavage - In centrolecithal eggs, the cleavage is restricted to

the peripheral cytoplasm of the egg. Eg: insects.

Laws of cleavage

Apparently there are several cleavage patterns. However, all cleavages follow a common procedure. The cleavages are governed by certain basic principles or laws.

1. Sach’s laws - These laws were proposed by Sach in 1877.

i) Cells tend to divide into equal daughter cells

ii) Each new division plane tends to intersect the preceding plane at right angles.

2. Balfour’s law (Balfour 1885) - “The speed or rate of cleavage in any

region of egg is inversely proportional to the amount of yolk it contains”.

DIFFERENT TYPES OF BLASTULA

The blastula of various groups of animals differ in form and structure depending upon a variety of factors such as the size of the the amount and distribution of yolk etc.

The following categories of blastulae have been recognized in different groups of animals.

1 COELOBLASTULA

It is a hollow blastula containing a large spacious blastocoel. usually, the blastocoel is f illed with a f luid containing mucopolysaccharides. The blastula resulting from holoblastic equal ceavage, as in the case of echinoderm s and Amphrbxus, is called equal coeloblastula . ln this case, the blastoderm is single layered.Holoblastic unequal cleavage, as in frog, results in unequal coelobtastuta. lt has a blastocoel displaced towards the animal pole

and a multilayered blastoderm.

2. Stereoblastala

This type of blastula is composed of an aggregate of larger sized and relatively lesser number of cells without or with extremely small blastocoelic space in the centre. Stereoblastula occurs in a

variety of animals such as insects, some worms like Nereis, mollusks like Cripidula, gymnophionan

amphibians and certain fishes

Discoblastula

Discoblastulonsists of a disc - shaped mass of blastomeres overlying a large yolk mass . This blastula is the resutt of meroblastic discoidal  cleavage as in most fishes, reptiles and birds.

There is no blastocoel, instead a slit like cavity called subgerminal cavity appears in between the blastoderm and the yolk mass.

Blastocyst:It is ihe blastula stage of mammals, it consists of a hollow spherical vesicular blastula, containing an inner cell mass at the animal pole. The embryo proper develops from the inner cell mass. The outer single layer of cells which encloses the blastocoel is called the trophoblast. The trophoblast establishes relations with uterine wall and helps in nutrition of the developing embryo

Fertilization in Frog

Fertilization is the fusion of sperm with egg resulting in the formation of zygote. It is charactetized by the following events.

1. Fertilization is external.

2. It is monospermy ,i.e. only one sperm fuses with the egg.

3.Thefertilized egg rotates in such away that the animal hemisphere goes above.

4.The jelly coat swells and increases in thickness.

5. The second meiotic division is completed resulting in the release of the secondpolar body'

6. The sperm enters the egg in the animal hemisphere at an angle of 40⁰ from the centre of animal pole.

7.Immediately after the entry of the Sperm into the egg, the vitelline membrane becomes elevated.This membrane is now called fertilization membrane. The space between this membrne and the surface of the egg is called perivitelline space filled with a fluid called perivitelline fluid. In this fluid, the fertilized egg can rotate freely.The rotation of the egg is inevitable for the normal process of development.Immediately  after fertilization, the black pigmented Animal pole placed above and the yolk-laden vegetal pole below.

8. Before the release of egg into the water' the jelly coat remains thin. As the egg is released into the water, the jelly coat absorbs water and begins to swell until the thickness of the jelly becomes twice the diameter of the egg.

9. The second maturation division is completed immediately after fertilization. As a result, the fertilized egg releases the second polar body.

10. The egg pronucleus and sperin pronucleus fuse together to form the zygotic nucleus. This process is called amphimixis.

11.On one side just below the equator,a crescent like area appears; it will be grey in colour. This area is called grey crescenl lt appears opposite to the point of sperm entry.The region of the grey crescent will become the posterior side and the opposite region will become the anterior side of the future embryo. This leads to the formation of a definite bilateral symmetry in the fertilized egg. .The unfertilized egg is radially symmetrical.

12. The sperm penetrates the egg perpendicular to the cortex. After penetration, the sperm moves in the cortex perpendicularly, along the radius ofthe egg. This path of the sperm is marked by pigment granules. This path ofthe sperm in theEgg cortex,is called penetration path.After crossing the cortex ,the sperm changes its direction and moves towards the egg nucleus.This changed path is also marked by pigment granules and is called copulation path.

Grey Crescent (Gray Crescent)

1. Grey crescent is a crescent-rike and grey coroured area developing on the surface of amphibian egg oppositc to the  point of sperrn entry.

2. It is a surface feature developing as a result of cyto plasmic movements stimulated  by the sperm entry in the egg.

3. It appears just above the margin where the yellow-white vegetal pole material merges with the darkly pigmented animal pole material.

4. It appears on the ,surface of the egg opposite to the point of sperm entry.

5. Grey crescent marks the future dorsal side of the embryo.

6. The first cleavage bisects the grey crescent into two equal  halves and this plane represents the future median plane of the embryo.

7.The formation of grey crescent, thus fixes up the final symmetry of the egg and the future embryo

8. In the gastrula, the grey crescent materials are located on the dorsal lip of the blastopore.

9. The grey crescent materials function as the organizer because, when it is- removed from the embryo, the embryo fails to develop further. At the same time when  a normal embryo is grafted with another grey crescent, two embryos develop.

10.In the late gastrula,grey crescent materials are incorporated in to the chorda mesoderm..

Cleavage of fertilized egg in Frog.

In frog’s egg the cleavage is holoblastic and unequal. The cleavage occurs as follows.

1. The first cleavage plane is meridional. Initially, a furrow appears at the animal pole. It gradually extends towards the vegetal pole of the egg. It cuts the egg through its median animal-vegetal polar axis and results in two equalsized blastomeres.

2. The second cleavage furrow is again meridional. It bisects the first cleavage furrow at right angles. It is a holoblastic cleavage affecting both the blastomeres of the first cleavage. It results in the formation of four blastomeres.

3. In the next stage a latitudinal furrow is formed above the horizontal furrow nearer to the animal pole. Such a furrow is due to the influence of yolk concentration in the vegetal pole. The latitudinal furrow uniformly affects all the blastomeres. It results in the formation of eight blastomeres. Four of them remaining in the vegetal pole are large. They are named as macromeres.

Another four blastomeres remain in the vegetal pole. They are named as micromeres. The micromeres are smaller in size than the macromeres.

4. The fourth set of cleavage planes are meridional and holoblastic. They are unequal. They divide yolkless micromeres more rapidly than yolk-rich macromeres. These cleavages result in the production of 16 blastomeres.

5. As a result of further cleavages, a ball of several small blastomeres result. A closer observation reveals that, while the blastomeres above the equator are small and remain as micromeres, the blastomeres of the vegetal pole remain progressively larger. The larger blastomeres are called the macromeres.

At the final stages of cleavage the embryo acquires a characteristic, mild, oblong shape. In this stage it is called the morula. The morula initially contains a shallow cavity called the blastocoels. Gradually the blastocoelic space increases into a large cavity occuping the middle of the blastula. However the blastocoel mostly remains in the animal pole region in the middle of the micromeres.

The blastomeres gradually adhere to each other, and arrange themselves into a true epithelium called the blastoderm. The blastoderm remains two cell thick in the animal pole. The embryo having a fluid-filled blastocoele and blastoderm is called the blastula.

It has been reported that around 12th cleavage the blastula possesses about 4096 cells. The blastula moves to the next stage, namely gastrulation at a stage in which it has about 20,000 cells. The ultimate blastula is a ball of blastomeres which have to form different embryonic body layers and organs of the body. The fate of each and every blastomere has been observed and marked.

Gastrulation in frog embryo

The process of gastrulation is a continuous activity succeeding, cleavage. During this process the blastodermal cells begin to move. They wander and occupy their prospective organ forming zones. During this movement at one region on the blastula, the cells wander inside and occupy the blastocoelic cavity.

At a specific region below the equator the blastoderm cells assume an elongated bottle like shape. They move toward the interior of the blastula. As the cells move further inside, an invagination happens. A deepening of the invagination results in a cavity called the archenteron or gastrocoel.The opening of the archenteron on the surface of the blastula is called the blastopore.

The blastopore gradually assumes a crescentic shape. Finally it becomes circular. The region dorsal to the blastoporal opening is called the ‘dorsal lip’. The lower edge may be called the ‘ventral lip’.

 The surface cells representing several prospective zones of the embryo begin to wander inside through the blastopore. These inwandering of cells is termed as involution.

Initially, the first pharyngeal endodermal cells undergo invagination over the dorsal blastoporal lip. These cells move to the interior. They are followed by other cells. The inwandering cells gradually occupy the region of the blastocoel.

Thus the blastocoelic cavity gets reduced. A new cavity among the involuted cells results. It is called the gastrocoel.  The gastrocoel later becomes the archenteron. The interior region of the archenteron gradually transforms into the pharyngeal region. This region remains as the foregut. The mesodermal and endodermal cells gradually occupy their positions.

The inward movement of the exterior cells through the blastoporal region is called involution.

The involution results in the positioning of chordamesodermal cells and pharyngeal endodermal cells.

The mesodermal cells occupy the region between inner endodermal and outer ectodermal cells.

While the exterior chorda-mesodermal cell involute inside, their place is taken up by the ectoderm. The expansion of the ectoderm is due to epiboly. Epiboly causes overlapping or ‘the roofing over’ of the gastrula by the ectoderm.

The blastopore is gradually covered by certain endoderm cells. The closing cells of the blastopore constitute the yolk-plug. Gradually the yolk-plug withdraws to the interior and the blastopore gets reduced into a narrow slit.

The process of gastrulation converts the blastula into a spherical, bilaterally

symmetrical, triploblastic gastrula. Gradually the gastrula undergoes the process of tubulation or neurulation to become a neurula.

Neurulation

The process of neurulation is the formation of a neural tube. However during this process mesoderm and endoderm also undergo differentiation. During neurulation the embryo lengthens along the anteroposterior axis. The dorsal side of the gastrula is lined by ectodermal cells. The presumptive area of the nervous system gets differentiated from the rest of ectoderm. It remains as medullary plate or nerual plate. The neural plate later thickens and it gets raised above the general level as ridges called neural folds. In the middle of the neural fold a neural groove appears. The neural groove deepens inside. The neural folds above the groove. The neural groove gets converted into a neural tube. This tube gets detached from the surface. The neural tube remains as the prospective nervous system. The embryo at this

stage is called the neurula.

During neurulation, the tubulation of chorda-mesoderm and tubulation

of endoderm also happen.

Development of Eye:

The eye is a photoreceptor. It is an ectodermal derivative. Its development begins even at the gastrulation stage. However the first sign of eye formation appears with the development of two optic vesicles from the lateral walls of the embryonic diencephalon.

Formation of optic cup

The eyes develop as two lateral outgrowths of the prosencephalon called optic vesicles. The cavity of the optic vesicle is called optocoel.

The connection of the optic vesicle with the brain becomes a narrow stalk like

structure called optic stalk.

 The optic stalk becomes connected with the ventral side of the optic vesicle rather than at its centre.

The optic vesicles extend outward and reach the ectoderm. The wall of the optic vesicle next tothe ectoderm is gradually flattened and later invaginates to form a doublewalled cup called optic cup.

The optic cup consists of two layers. The inner layer (derived by invagination)

gives rise to the nervous region, the retina.

 The outer layer will be a thin, black pigmented layer (tapetum - nigrum) for the absorption of light.

 Initially the opening of cup is very large. Soon its rim bend inward and converges, so that the opening is reduced. This opening is called the pupil.

The rim of the optic cup surrounding the pupil becomes the iris. Later on, large amount of pigment is deposited in the outer epithelial layer of iris.

 Agroove extends along the ventral side of the optic cup. It is called the choroid fissure. It extends to the middle of optic stalk. It serves for the entry of blood vessels and mesenchyme cells into the posterior chamber of eye.

The retina develops a membrane on its inner most surface called the internal limiting membrane.

 The outermost cells of the neurosensory retina differentiate into rods and cones. The inner cells of the retina differentiateinto neuroblasts or nerve cells.

Development of lens :

When the lateral surface of the growing optic vesicle comes in contact with the ectoderm it gives off stimulus of some kind, which causes the ectodermal cells to elongate, forming a disc shaped thickening. This is called the lens placode or lens rudiment.

 It curves into a cup and finally separates from the ectoderm. The free edges of the cup fuse to form a globular hollow lens vesicle. The lens vesicle comes to lie in the cavity of the optic cup.

The cells of the inner side of the lens vesicle elongate, become columnar and are finally transformed into long fibres. Their nuclei degenerate and cytoplasm becomes hard and transparent making it refractile. These cells are called lens fibres.

 The outer layer of the lens remains unchanged and becomes the lens epithelium. The junction between the lens fibres and the lens epithelium represents the growing point of lens. Here the epithelial cells are continuously transformed into lens fibres.

When the lens is formed, the free margin of optic cup touches the edges of the lens and grows in front forming iris. Thus lens hangs in the opening of optic cup.

Soon after the development of lens the overlying ectoderm closes over and differentiates to become the cornea.

 It is continuous with the skin. The transformation of the skin into cornea is caused by an induction arising from the optic cup and lens. The ectodermal cells covering cornea form an extremely thin, transparent membrane. This is known as conjunctiva of the eye ball.

 In adult this becomes continuous with the inner lining of upper and lower eyelids. The space between lens vesicle and the overlying presumptive anterior epithelium of cornea represents the anterior chamber. It contains cellular material called anterior vitreous body.

The choroid and sclerotic coat of eye develop from the mesenchyme cells accumulating around the eye ball. The interior layer of mesenschyme cells give rise to a net work of blood vessels surrounding the pigmented retinal layer and is called choroid coat.

 The outer layer of mesenchyme form fibrous capsule, the sclerotic coat or sclera around the eye. The sclera provides protection to eye and the eye muscles.

The ectoderm from above and below the original lens placode region grows out as two folds. These folds grow over conjunctiva and come to touch each other forming a complete layer of ectoderm. At a later stage these folds separate along the line of fusion to form the regular upper and lower eye lids.