<P> 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 . The primitive streak extends through this midline and creates the left -- right and cranial -- caudal body axes , and marks the beginning of gastrulation . This process involves the ingression of mesoderm progenitors and their migration to their ultimate position , where they will differentiate into the mesoderm germ layer that , together with endoderm and ectoderm germ layers , will give rise to all the tissues of the adult organism . </P>
the region known as the primitive streak is the site of
gastrulation and initiate germ layer formation