The bilaminar germ disk differentiates itself further into a trilaminar embryo, in that the cells flow in over the primitive streak between the two already existing germinal layers and so form the third embryonic germinal layer (mesoblast/derm). This phenomenon is also termed epithelio-mesenchymal transition (gastrulation in lower vertebrates) (stade 6-7). During this period the embryo experiences profound alterations. Afterwards, one speaks of the dorsally lying ectoblast/derm (and no longer of an epiblast/derm), from intermediate mesoblast/derm, as well as from ventrally lying endoblast/derm, which replaces the hypoblast. In order to have a better overview, the developments of the third week should be divided into several phases. One must keep in mind, though, that these do not always follow each other - they can just as easily take place concurrently.
Formation of the primitive streak
Beginning with the third week ectoblast cells migrate through the primitive streak and form the mesoblast. At this stage the embryonic disk is oval -shaped and the ectoblast is bathed in amniotic fluid.
Viewed dorsally the wide end represents the rostral (front) region. The narrower end forms the caudal region. At this point, one can speak of right and left halves of the embryo if one lays a longitudinal (rostro-caudal) axis through the embryo.
From the 17th day (stage 6) a thickening of the embryonic disk begins around the median line along the rostro-caudal axis. This median structure (primitive streak) lengthens until it occupies roughly half the embryo (stage 7). The primitive streak arises thanks to the proliferation and migration of epiblast cells in the direction of the embryonic disk's median line.
After the 19th day (stage 7), the primitive streak grows through the addition of cells at its caudal end. At the anterior end, a groove forms in the ectoblast (primitive groove). The cranial region is strengthened by the epiblast cells and so forms the primitive pit (stage 8) with the primitive node (stage 7) (Hensen's node in birds). The head of the embryo will form at the extremity of the embryonic disk near the primitive pit.