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

Genesis of the notochord

Around the 19th day cells that invade into the primitive node region and migrate along the median line cranially form the chordal process 7 (also known as the axial process). One can compare this phenomenon with the sliding of a finger into a glove. Thanks to the transparency of the ectoblast this cell migration can be observed in the embryos of experimental animals.
The chordal process grows longer through proliferation of the primitive node cells at its front end up to the prechordal plate 6. At the same time the primitive streak recedes back into the caudal region 6.

Fig. 5
The red arrow shows schematically the migration direction of the epiblast cells coming from the primitive node to form the notochordal process and later the notochord.

Fig. 5a
Schematic diagram showing the genesis of the chordal process at the 19th day through the invagination of epiblast cells coming from the primitive node.

Fig. 5b
Cut along C.

Fig. 5c
Schematic diagram showing the genesis of the axial canal through invagination of epiblast cells that migrate into the region of the primitive node (on roughly the 21rst day).

Fig. 5d
Cut along C

On the 23th day the chordal process consists of chordal mesoderm and a central axial canal 7. At this point in time, the chordal process fuses with the endoderm lying below it. While the chordal process and endoderm are merged, for a short time (approximately one day), the amniotic cavity communicates with the umbilical vesicle, via the so-called neurenteric canal 8 . The chordal tissue that thereby goes over into the endoderm becomes thus the chordal plate 8-9 .

Fig. 6a
The notochord has separated itself from the endoblast. At the caudal end of the notochord, in the region of the primitive node, thanks to the neurenteric canal, a temporary communication takes place between the umbilical vesicle and the amniotic cavity (arrow).

Fig. 6b
Cut along C

On the 25th day 9 the chordal plate cuts itself off from the entoderm, which then fuses again, and forms a complete cord: the notochord. This is in the middle of the mesoderm, between the ectodern and endoderm, and plays a role in the induction of the neuroectoblast that lies over it. Moreover, the notochord plays a role in the genesis of the vertebral body and becomes the nucleus pulposus in the intervertebral disk.

Fig. 7a
At around the 20th day the chordal tissue bonds with the endoderm and thus forms the chordal plate.
Between the 22nd and 24th day it cuts itself off from the endoderm (the endoderm fuses again) in order to form a complete cord: the notochord. This finds itself in the middle of the mesoderm, between the ectoderm and endoderm.
Fig. 7b
Cut along C

Fig. 7c
The endoderm, under the notochord, comes together again. At this point the neural groove has partly fused into becoming a neural tube.
Fig. 7d
Cut along C