Location of the epiblast cell target and the development of the primitive streak
Depending on their place of origin and the timing of the invagination, the epiblast cells migrate away from the primitive streak in three main directions.
It is assumed that the extraembryonic mesoblast comes from ectoblastic cells passing through the posterior segment of the primitive streak.
The intraembryonic mesoblast, on the other hand, arises from cells migrating through the middle and cranial segment up to the 4th week.
The ectoblast cells that converge to the primitive node engender the paraxial mesoblast, the notochord, the prechordal plate, the endoblast and the medial part of the somites (in animal experiments as determined with the help of marked cells).
At around the 19th day (stage 9) the primitive streak extends over half the whole length of the embryo, but recedes with advancing gastrulation and is shifted caudally.
In the 4th week its length is approximately only 15% of that of the whole embryo. The primitive streak is confined to a region termed the caudal eminence.
On the 29th day (stage 11) it disappears completely. Remainders of it can lead to a sacro-coccygeal teratoma.