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.
The fully formed and freshly laid hen's egg is large. It is 3cm. in diameter and 5cm. in length. It contains enarmous amount of yolk. Such egg is called macrolecithal egg. The egg is oval in shape. The ovum contains a nucleus. It is covered by yolk free cytoplasm. It .is 3mm. in diameter. It is seen on the animal pole. The entire egg is filled with yolk. This yolk has alternative layers of yellow and white layers. They are arranged concentri­cally arround a flask shaped structure called latebra. Below the blastodisc the neck of latebra expands. This is called nucleus of pander. Yellow yolk got its colour because of carotenoids White yolk layers are thin and yellow yolk layers are thick. Yolk is a liquid. It contains 49% water and 33% phospholipids 18% proteins, vitamins, carbohydrates.
The entire ovum is covered by plasma membrane. It is called plasmalemma. It is lipoprotein layer. This is ovum is covered by egg mem­branes.
Primary membranes: These membranes develop between oocyte and follicle. The primary membranes are secreted by follicle cells. It is called vitelline,membrane is come from two origins. Inner part is pro­duced by ovary. Outer part is from the falopian tube. (This is stated by Balinsky)

Secondary membranes: Oviduct secretes secondary mem­branes. Above vitelline membrane albumen is present. It is white in colour and it contains water and proteins. The outer layer of albumen is is called thin albumen. The middle layer of albumen is thick. It is called thick albumen, or dense albumen. The inner most albumen is very thick. It develops into chalazae. The chalazae are called balancers. They keep the ovum in the centre.

Shell membranes: Above the albumen two shell membranes are present. Towards the broad end of egg, in between the shell membranes an air space is present. This air space is formed when egg is laid cooled from 60°C to lesser temperature.
Shell: Above the shell membranes a shell is present, it is porous in nature. It is calcareous. This porous shell is useful for exchange of gases. In a freshly laid hen's egg shell is soft. Very soon it becomes hard.
The Study of the developmental stages of an organism is called If we observe the embryos of different animals, there is a similarity. This similarity tells us that there is a relationship between the animals. The embryological evidences show support to organic evolution.
All multi cellular organisms begin their life as a single celled stage, namely Zygote. It undergoes cleavage to produce the first embryo, called morula. It develops into a single layered second embryo, called blastula. This embryo develops into a third embryo called gastrula. This embryo develops into adult. The Zygote represents the unicellular stage. Morula and blastula represent the colonial protozoan stages. Whereas gastrula represents the coelenterate stage. The embryos of organisms differ after the gastrula stage. The sequence of embryos shows that every multi cellular organism passes through the above stages representing their ancestors.
Zygote -------> Morula-------> Blastula -------> gastrula ------->adult.
(protozoan (colonial protozoan (coelenterate stage) stage) stage)
Von Baer proposed these principles by studying the embryology of Fish, Frog, Tortoise, Pigeon, Chimpanzee and Man. The early embryos of above animals resemble with each other closely. That it is impossible to separate if the embryos were mixed. But the embryos differ in the final stages clue to the formation of specialized characters. The similarity of early embryos tells that the above animals have common ancestors. The embryological principles are
  1. General characters appear in the early embryos.
  2. The special character appear in the last embryos.
  3. The embryos of closely related individuals are almost similar up to the end with small differences.
  4. The embryos of one organism resembles to the embryos o f its ancestors but not with adults.
It was proposed by Earnest Haeckel . The Study of the sequence of embryos or life history of an organism is called ontogeny. The evolutionary history of an individual is called phylogeny.
Biogenetic law states that every organism recapitulates its ancestors through the embryos. In other words the ontogeny of an individual repeats its phylogeny. The biogenetic law is also called recapitulation theory. Earnst Haeckel is considered as the father of embryology. Von Baer is considered as the father of modern embryology.
Biogenetic Law - Examples:
I. Tad pole larva of Frog:
The tad pole larva resembles fish both externally and internally. It contains two chambered heart, respires with gills like Fish. Later tad pole larva metamorphoses into adult Frog. Frog recapitulates its closet ancestor, fish through the tad pole larva. It also indicates that the Frog was evolved, from fish.
ii. Cater pillar larva of Butter fly:
This larva resembles to annelid. Butterfly recapitulates its closest ancestor, annelid through the caterpillar larva. It alsc tells us that the Butter fly was evolved from annelid.
iii. Development of ‘4’ chambered heart in the embryos of Birds and Mammals:
ontogeny repeats phylogeny18
In the embryos of Birds and Mammals, the heart is ‘2’ chambered, then ‘3’ chambered and incompletely ‘4’ chambered before developing into complete ‘4’ chambered heart. This shows that the Birds and Mammals recapitulate even the development of heart also.
Likewise the recapitulation is also observed in the development of brain, aortic arches, kidneys etc.
iv. Temporary embryonic nonfunctional organs:
The embryos consist of some temporary embryonic organs. These are present only in their ancestors but absent in adults. Such organs are called temporary embryonic vestigial organs. But these disappear when the embryo develops into adult. Some of the temporary embryonic vestigial organs are
  1. Formation of gill slits in the embryos of reptiles, birds and mammals.
  2. Formation of teeth in the embryos of pigeon and whale.
  3. Formation of limbs in the embryos of snakes.
  4. Formation of tail in the embryo of human beings.
These structures are of no use in the embryos. But their presence explains the biogenetic law.
These are the remains impressions of past lived organisms.

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