Gastrulation in Chick

Cleavage in the fertilized egg occurs during its passage through the oviduct to cloaca of the hen. By the time it is laid meroblastic cleavage has resulting in the creation of a various layered W-like blastoderm in which two distinct regions can be accepted:

1. The clear central circular area pellucida separated from the fundamental yolk through a space called the sub-germinal space or cavity and
2. The peripheral area opaca wherein the cells of the lower layer are in intimate contact along with the yolk making this region dark and opaque.

The inner border of area opaca directly adjoining the area pellucida is the marginal zone and its remaining peripheral part is termed as the germ wall. A thickened region of marginal zone because of great density of cells indicates the future posterior side of the embryo. In chick, gastrulation is a prolonged process. It is started soon after the beginning of incubation after laying of the egg and is completed in about 4 days. The actual process of gastrulation is preceded through some pre-gastrula movements of certain cells resultant in their separation from the blastoderm and formation of a lower layer called the hypoblast. Though, Most of the cells remain in the upper layers of the blastoderm which now constitute the epiblast.