In amniotes the developing embryo in order to grow properly foetal membranes are formed.
When few membranes are produced by mother, it should take more care for their survival. If the number are more, care will be less. In amniotes when the developing embryo is enveloped, by extra embryonic membranes, which will give scope, for developing embryo, the extra em­bryonic membranes are chorion, amnion, yolk sac, allantois.
In the development of chick these membranes will develop from orginal blastoderm, the central part of blastoderm will give embryo proper, the marginal blastoderm will give extra embryonic membranes amnion and chorion will develop from somatopleurae, yolk sac and allontois, will develop from spiafichnopleurae.
Amnion & Chorin: In the development of embryo amnion and chorion are closely associated, Amnion is bag like covering over'the embryo, it separates the embryo from internal environment, Amnion is developed from somatopleuric amniotic folds. These folds are head fold, lateral folds and tail folds.
  1. At about 30 hours of incubation, in front of the head of embryo a head fold is developed, it is called amniotic head fold.
  2. At about third day of incubation amniotic tail fold is developed. It grows opposite to head fold.
  3. Mean while lateral folds will develop, they grow dorsomedially.
  4. After some time head fold, lateral folds, and tail fold will fuse near posterior end of a embryo.
  5. At 72 of incubation they are still not fused. They show an opening called amniotic umblicus, afterwards they unite.
  6. After their union at the point of union "sero-amniotic raphae" is present. It is a fold.
  7. Because of this union outer chorion inner amnion will form, because it is developed from somatopleure. In chorion ectoderm is present out side and mesoderm is present inside. In amnion ectoderm is inside, mesoderm is out side. Hence the space between amnion and chorion is called exocoel or extraembryonic coelome.
Functions of chorion:
  1. The extra embryonic coelome is filled with a fluid. It gives protection to the developing embryo.
  2. This coelome gives space, for developing allantois.
  3. Chorion combines with allantois and acts as a respiratory organ.
Functions of Amnion:

Amnion is sac like structure around embryo. It contains amniotic fluid. It will protect embryo from mechanical shocks and dessications.
It protects the embryo when the egg is laid. It gives artificial aquatic environment for growth of embryo.
Yolk sac:
At 16 hours of incubation, yolk sac makes its appearence.
It develops from Splanchanopleurae Splanchanopleurae contains Endoderm and mesoderm layers.
The Splanchanopleuraeinstead of forming a close gut, it will grow over yolk, and becomes yolk sac.
The primitive gut is present above the yolk. This yolk region is in contact with midgut. Finally the yolk sac is communicated with midgut through an opening.
Functions of Yolk sac:
It digests the yolk, and the digested food will be circulated through blood to the developing embryo. Hence yolk sac is considered as a nutritive organ of the embryo.
In chick it develops from the ventral part of caudal end of the hindgut. It develops at third day of incubation. It develops from SplanchanopleuraeThisSplanchanopleuraecontains endoderm and meso¬derm, the allantois grows rapidly, and occupies the entire exocoel. The mesoderm of thechorionand mesoderm of allantois will unite. It forms chrio allantoic membrane.

Allantois is connected to the hindgut, and is called as allantoic stalk.
As the embryo is growing the allantoic and yolk stalk are brought together. Their mesodermal layers will unite. It is called umblical stalk. It is covered by somatic umblicus.
Functions of Allantois:
Allantois is richly vascularised. Hence it works as respiratory organ.
It stores nitrogenous waste material of the embryo.
In later development the allantoic circulation will absorb calcicum from the shell. This calcium is used in construction of bones in young ones.
Allantois absorbs calcium from shell. Hence the shell becomes thin. It helps in rupturing the shell during hatching.
The mesoderm is proliferated by primitive streak. It is formed as two layers. In front of the primitive streak an area without mesoderm is present. It is called proamnion. After 48 hours of incubation the proamnion is also occupied by mesoderm. The mesoderm is divided into dorsal and intermediate and lateral mesoderms.

The notochordal cells arrange themselves to form a cylindrical, rod called notochordal process. It will begin at hensen's node and it slowly grows. Because of its growth the primitive streak is slowly reduced. By the end of gastrulation the primitive streak is reduced and incorporate into tail bud.
The dorsal mesoderm is located on either side of notochord. It is divided into segments. They are called somites. The first pair of somites will form after 21 hours of incubation. Afterwards, for every one hour one pair of somie will add. The 24 hours old embryo contains four pairs of somites.
The intermediate mesoderm connects the dorsal mesoderm with lateral mesoderm as a stalk. Afterwards it undergoes segmentation and give kidneys.
The lateral mesoderm extends on periphery of embryo, it is divisible into extra embryonic and embryonic mesoderms. This lateral mesoderm will split into two layers. The upper layer is called somatic mesoderm and inner layer is called splanchnic mesoderm. Ectoderm and somatic mesoderm will be called somatopleure. The splanchnic layer and endoderm will be called splanchnopleure. In between the two layers of mesoderm the space is called coelome.
Thus at the end of gastrulation specific organ forming areas started to develop.
The second step in gastrulation is the formation of primitive streak. At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a thickened area. It starts eight hours after incubation. The thickening is because convergence of cells of blastoderm towards the centre. Usually in the early stages the primitive streak is short and broad. It is called primitive streak. It gradually extends to the middle of blastoderm. At eighteen to nineteen hours of incubation, primitive streak is well developed. It is caikd definite primitive streak. Along the middle line of primitive streak a narrow furrow is develop called Primitive groove. The edges of groove are thick. They are called primitive folds. At the anterior end of groove a mass of closely packed cells will be present. It is called "hensen's node" or primitive knob. In the centre of this node a pit is present. It is called primitive pit. It represents the vestige of neurenteric canal. The primitive streak elongates along with this, area pellucida will also elongates. As the primitive streaks growing the cells from this region will invigilate into space between epiblast and hypoblast This process is called immigration. The immigrated cells will become prechordal plate, notochord, and mesoderm. Anterior to primitive streak the mesoderm cells will not migrate to primitive streak. This mesoderm free area is called proamnion. At this place head will develop.
Formation of endoderm: Endoderm of hypoblast develops as a single layer of cells in side of blastocoel. After the formation of endoderm, upper layer is called epiblast. There are different theories to explain the formation of endoderm.

Infiltration theory: This was proposed by Peter in 1923. According to this theory some cells in blastoderm which are loaded with yolk will fall into blastocoel. It starts from posterior end of blastoderm. From there the cells migrate forward one behind another and endoderm is formed.

Delamination theory : It was proposed by Spratt in 1946. Blastodenr. is two or three layered thick. The lower layer will separate-fcoijl the upper layer by splitting and the lower layer is called endoderm, upperr layers are called ectoderm. In between ectoderm and endoderm blastocoel is present.
Theory of involution: In 1909 Peterson Proposed this theory. According to this theory a slit like opening at the posterior side of blastoderm forms. Through this opening the blastoderm cells will role into the primary blastocoel. It forms an endoderm.
Theory of invaginaton: This was proposed by Jockobson in 1938. According to this theory the posterior end of blastoderm will invaginate in blastocoel as a small pocket. This becomes endoderm. In this way endoderm is formed.

Laying of the egg: Between 9 A.M. and 3 P.M., the egg is expelled from the cloaca of hen. At the time of laying formation of endoderm is completed. For further development it is to be incubated.

Incubation: When the egg is laid, the development is stopped. For further development it is to be kept at 38°C. This is done by hen by sitting over the egg. This is called incubation. Artificially eggs are incubated in incubators. For the hatching of egg 21 days are required.
In the upper region of oviduct fertilization will takes place. One sperm will penetrate into hens egg and fertilizes with the egg. The fertilized egg will travel through oviduct. It takes nearly 22 hours. Hence the early development of egg will take place in oviduct.

Cleavage: Cleavage is restricted to blastodisc. The presence of great quantity of yolk the cleavage is restricted to blastodisc. Such cleavage is called meroblastic or discoidal cleavage.
1. Cleavage: After five hours of fertilization the first cleavage will appear. It is confined to the centre of blastodisc. cannot completely divide the blastodisc. Blastomeres are not formed.
2. Cleavage: It takes place at right angles to first cleavage. Even because of second cleavage clear blastomeres are not formed.
3. Cleavage: It is vertical. It is in the two sides of first division. As a result of this division eight blastomeres are formed. But they do not show boundaries.
4. Cleavage: It takes place in such a way that eight central blastomeres and eight peripheral blastomeres will form. Only at this stage of division definite cells are formed. The central eight cells are completely separated from yolk.
After fourth cleavage the cleavages are irregular and a blastoderm is formed.
As the central cells are forming below these cells a horizontal cleft is formed. Because of this an upper layer of cells and a segmentation cavity is formed. This segmentation cavity is called blastocoel. These cells will undergo further division quickly. Hence above the segmentation cavity mass of cells will be present, in several layers. These cells have complete boundaries. The cells present towards the periphery are not separated from yolk. They are called marginal cells. This region is called zone of junction.
Area Pellucida & Area opaca:
The central cell mass of the blastoderm will be in four to five layers, they are lifted from the yolk. Hence the central part of the blastoderm is free from yolk. This region is transparent. It is called area pellicida. At the zone of junction the cells are in contact with that region is opaca. That region is called area opaca.
Shape: The 48 hours chick embryo will show this shape? (In mirror view). In this stage of development the head region shows prolific growth. Because of which the embryo shows this peculiar shape.
Flexure: In the head region bendings are seen. These bandings are called flexure. This bending is towards right and backwards. In 48 hrs. Chick embryo two flexures are seen. One is at the region of midbrain. Because of this fore and hind brain will become parallel. This is called cranial flexure.
In the posterior region of brain a cervical flexure is formed. This flexure turn the head towards right. This region will give neck of adult.

Torsion: The 24 hours chick embryo is flat. Its ventral surface is in contact with yolk. As the development proceeds the anterior end of embryo is turns towards right and hence the anterior part of embryo comes to lie on the left side of yolk. This twisting is called torsion. At the end of 48 hours chick twisting will reach the cervical flexure. It is nearly at 13th somite.
Brain: The brain, shows three regions. Fore brain shows telen­cephalon and diencephalon, the hind brain shows medulla oblongata.
Eye: The optic cup is two layered. Lens placode is started its development.

Ear: The invagination of optic vesicle is completed. It is connect­ed to ectoderm by endolymphatic duct.
Alimentary canal: Fore gut reaches 1/2 mm length.
Heart: It is 5' shaped. It is tubular. !t has not developed cham­bers sets.

Blood vessels: From the anterior end of heart two ventral aortae will arise. They run backwards and travel as dorsal aortae.

Kidney: Pronephrous is completely developed. Mesonephric tubules started development.

Somites: At 48 hours of incubations 25 pairs are seen. Mesonephric tubules will develop from intermediate mesoderm.
At 24 hrs. incubation period the chick embryo is oval in shape. The primitive streak is seen. From the hensen's node a notochord is developed.
In the 24 hrs. chick embryo four pairs of somities are present.
NEURALFOLDS: On either side of mid dorsal line the ectoderm becomes thick. It forms longitudinal folds. They are called neural folds. They include a space called neural groove, this neural groove is deep in anterior region, and the depth decreases gradually posterior region. Near the first somite region the folds will startrfusing. It will start after 24 hours of incubation. Posteriorly diverse. This area is called sinus rhomboidalis. In this region regressing primitive streak is developed.
GUT: At this incubation stage foregut is formed. Mid and hind guts are not formed. The opening of foregut into midgut is called anterior intestinal portal on either side of this the splanchanic mesoderm becomes thick.
MESODERM: It is present between ectoderm and endoderm. It occupies the entire blastoderm.

BLOOD ISLANDS: Around the area pellucida, area opaca is present. In this opaca region groups of mesenchyme cells will present. They are called blood islands. They produce embryonic blood vessels.
24hours chick embryo thumb18
AREA VASCULOSA & AREA VITELLINA: In the area opaca the region which contain blood islands, is called area vasculosa. The peripheral area without blood vessels is called area vitellina.


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