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.
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.
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.
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.
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.
Ear: The invagination of optic vesicle is completed. It is connected to ectoderm by endolymphatic duct.
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.
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.
- 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