Gastrulation is the term for establishment of trilaminar embryonic disc and formation of three primary germ layers: ectoderm, mesoderm and endoderm. Three important structures form during the 3rd developmental week: the primitive streak, the notochord and the neural tube.
The primitive streak appears caudally in the midline of the dorsal aspect of the embryonic disc. In the region of the primitive streak and its cephalic end (the primitive node), the epiblast cells move inward (invaginate) to form a new cell layer (intraembryonic mesoderm) between the epiblast and hypoblast. Some of the epiblast cells displace hypoblast, there by creating the embryonic endoderm. The cells remaining in the epiblast form the ectoderm. Cells migrating cranially through the primitive node form the notochordal process (prospective notochord), which defines the primitive axis of the embryo.

1. Cut edge of amnion
2. Primitive node
3. Primitive streak

1. Prechordal plate
2. Primitive node
3. Primitive streak
4. Cloacal membrane

1. Ectoderm
2. Mesoderm
3. Notochord
4. Endoderm
5. Prechordal plate
6. Primitive node
7. Allantois
8. Primitive streak
9. Caudal mesoderm
10. Amnion
11. Yolk sac cavity

The vertebral column forms around the notochord. The developing notochord induces the overlying ectoderm to form the neural plate e.g. the primordium of the central nervous system. The mesoderm layer and the notochord separate the ectoderm and endoderm layers entirely, with the exception of the prechordal plate cranially (future oropharyngeal membrane) and the cloacal membrane (future anus) caudally. The neurenteric canal is a small canal that temporarily connects the yolk sac and the amniotic cavity. The posterior wall of the yolk sac forms a small diverticulum (allantois) which extends into the connecting stalk.
The caudal end of the embryonic disc continues to supply new cells after the differentiation of germ layers in its cranial part.