Primitive streak

Proliferation and migration of cells lead to a local thickening of the epiblast in the caudal proliferation zone. This zone now extends cranially along the median plane to form the primitive streak. The higher local proliferation rate and the more medially ongoing cell movement causes the epiblast to thicken much more in the region of the primitive streak than in the lateral neighbourhood. Along the midline proper, a longitudinal deepening (the primitive groove) develops where cells leave the epiblast.
A ridge appears at the cranial end of the primitive streak, which forms the thickened proliferation zone: the primitive node (Hensen's node). Beneath this structure is a small, funnel-like deepened pit: the primitive pit. Cells migrate cranially from the primitive node under the epiblast, delivering the material for both the prechordal plate and the notochordal process. The notochordal process does not form a new layer between epiblast and hypoblast. Rather, it integrates in the plane of hypoblast cells, and pushes them aside. At S8, the primitive pit can deepen to form the notochordal canal. From S9-S10, the floor of the notochordal canal can break down and a neurenteric canal can be formed temporarily. It connects the amniotic cavity with the umbilical vesicle. The primitive node gives rise to cells of the axial structures, resulting in the prechordal plate, and the notochordal process giving rise to the chorda dorsalis. In addition, cells of the embryonic endoderm and of the medial halves of the somites have originated from the proliferation zone of the primitive node.