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.