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 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.


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