1

From the 17th day 6 the primitive streak forms an entry location for the epiblast cells to invade the underlying mesoblast and to proliferate there. Histological methods have shown that during their migration along the primitive groove, the epiblast cells form pseudopodia. They lose contact with each other thereby. This phenomenon of the inflow of cells to form the third embryonic layer is termed epithelio-mesenchymal transition 6-7 (gastrulation in the lower vertebrates) .

2

Depending on their origin and themoment of their flowing in, the epiblast cells migrate away from the primitive streak in various directions. The first cells that enter through the node and the primitive groove replace the hypoblast layer and form the definitive endoblast (origin of the future intestine and its derivates). At the same time, due to the migration of cells through the primitive node in the cranial direction, two further structures are formed: the prechordal plate 6, which is located cranial to the primitive node the notochordal process8the development of which will be treated in the next section. The largest proportion of these immigrated cells form a third germinal layer, theintraembryonic mesoblast 6. The mesoblast cells wander in all directions: laterally, cranially and caudally. This middle germinal layer lies between the definitive endoblast and epiblast. Exceptions are the cloacal membrane as well as thepharyngeal membrane, where the ectoderm and endoderm lie directly opposite each other 6. Cranial to the prechordal plate, mesenchyma cells of the embryonic disk will form thepericardium as well as the septum transversum 9 . At the caudal extremity the cloacal membrane forms the primordium of the future opening of the uro-genital tract and the rectum.

3

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.

4

5

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 .

6

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 theinduction 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.

7

Summary: the notochord determines the longitudinal axis of the embryo. It defines the future situation of the vertebral body and induces the ectoblast in its differentiation to become the neural plate.

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9