blastopore

Amniotes, like chickens and humans, will have developed a primitive streak, the precursor to the brain and skin, while fish and amphibians will have developed a spherical-shaped blastopore.

It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.

The blastopore persists as the anal opening of the adult.

The blastopore generally occupies a position corresponding to the posterior end of the body.

Then the large cells recommence the process of division and sink into the hollow of the sphere, leaving an elongated groove, the blastopore, on the surface.