the region known as the primitive streak is the site of
Primitive streak The presence of the primitive streak will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the second embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation.[1] The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes,[2] and marks the beginning of gastrulation.[3] This process involves the ingression of mesoderm progenitors and their migration to their ultimate position,[2][4] where they will differentiate into the mesoderm germ layer[1] that, together with endoderm and ectoderm germ layers, will give rise to all the tissues of the adult organism.