Blastoderm

A Blastoderm is the layer of cells formed at one pole of the yolky egg of birds. The yolk prevents the division from taking place through the egg, resulting in meroblastic cleavage during the many cleavage divisions. At the pole containing less yolk, the ovum divides and subdivides, forming the blastoderm, which gradually spreads around the yolk and forms the embryo. The blastoderm is comprised of two layers, the epiblast and the hypoblast, which enclose the fluid-filled blastocoel cavity. This embryonic stage is the avian equivalent of birds.