7.2 The trilaminar germ disk (3rd week)

• Introduction
  
• Formation of the primitive streak
• Genesis of the germ layers
• Genesis of the notochord
• Location of the epiblast cell target and the development of the primitive streak
• The bilaminar membranes
• Evolution of the mesoblast
  • The paraxial mesoblast and the differentiation of the somites
  • The intermediate mesoblast
  • The lateral mesoblast
• The intraembryonic coelom
• Induction of the neural plate - neurulation

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 (1).
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 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 11 it disappears completely. Remainders of it can lead to a sacro-coccygeal teratoma.

Fig. 8, Fig. 9
Schematic diagrams: dorsal view of the embryonic disk during the 3rd week. One sees the growth of the primitive streak and the genesis of the chordal process.

Fig. 10, Fig. 11
Schematic diagrams: dorsal view of the embryonic disk during the 4th week. To be seen are the back-formation of the primitive streak and the growth of the chordal process.
The primitive streak is confined to a region termed the caudal eminence and disappears at stage 11 (29 days).