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.
I.Cleavage : After five hours of fertilization the first cleavage will appear. It is confined to the centre of blastodisc. Itismeridional.lt cannot completely divide the blastodisc. Blastomeres are not formed.
II.Cleavage : It takes place at right angles to first cleavage. Even because of second cleavage clear blastomeres are not formed.
III.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.
IV.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