 The primitive streak is a structure that forms in the blastula during the earlier stages of avian, reptilian and mammalian embryonic development. It forms on the dorsal back face of the developing embryo, toward the caudal or posterior end. The presence of the primitive streak will established by lateral symmetry, determined 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 progenters 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. It is important that the chicken embryo can be easily manipulated, most of our knowledge about the primitive streak comes from avian studies. The marginal zone of a chicken embryo contains cells that will contribute to the streak. This region has a defined anterior to posterior gradient in its ability to induce the primitive streak, with the posterior end having the highest potential. The epidlast, a single epithelial layered last of disk, is the source of all embryonic material in amnotes and some of its cells will give rise to the primitive streak. All cells in the epidlast can respond to signals from the marginal zone, but once a given region is induced by these signals and undergo streak formation, the remaining cells in the epidlast are no longer responsive to these inductive signals and prevent the formation of another streak. Underlying the epidlast is the hypodlast, where the extra embryonic tissue originates.4 in the chick, the absence of the hypodlast results in multiple streaks, suggesting that its presence is important for regulating the formation of a single primitive streak. In mites, this structure is known as the anterior visceral endodermivate.