Around the 19th day cells that invade into the primitive node region and migrate along the median line cranially form the chordalprocess (stage 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 prechordalplate (stage 6). At the same time the primitive streak recedes back into the caudal region.
On the 23th day the chordal process consists of chordal mesoderm and a central axial canal (stage 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 (stage 8). The chordal tissue that thereby goes over into the endoderm becomes thus the chordal plate (stage 8-9).
On the 25th day (stage 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.
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.