In amniotes the developing embryo in order to grow properly foetal membranes are formed.
- At about 30 hours of incubation, in front of the head of embryo a head fold is developed, it is called amniotic head fold.
- At about third day of incubation amniotic tail fold is developed. It grows opposite to head fold.
- Mean while lateral folds will develop, they grow dorsomedially.
- After some time head fold, lateral folds, and tail fold will fuse near posterior end of a embryo.
- At 72 of incubation they are still not fused. They show an opening called amniotic umblicus, afterwards they unite.
- After their union at the point of union "sero-amniotic raphae" is present. It is a fold.
- 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.
- The extra embryonic coelome is filled with a fluid. It gives protection to the developing embryo.
- This coelome gives space, for developing allantois.
- Chorion combines with allantois and acts as a respiratory organ.
Amnion is sac like structure around embryo. It contains amniotic fluid. It will protect embryo from mechanical shocks and dessications.
Allantois is connected to the hindgut, and is called as allantoic stalk.
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.
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 u.in.It 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.
- FRESHLY LAID HEN'S EGG
- CHICK EMBRYO AT 24 HOURS
- CHICK EMBRYO AT 48 HOURS
- GASTRULATION IN CHICK-I
- GASTRULATION IN CHICK-II - FORMATION OF ENDODERM
- GASTRULATION IN CHICK-III - FORMATION OF PRIMITIVE STREAK & MESODERM
- GASTRULATION IN CHICK-IV - DEVELOPMENT OF MESODERM AND COELOME
- CHICK: EXTRA EMBRYONIC MEMBRANES