Axial mesoderm

The first cells leaving the epiblast exit via the primitive node and enter the space ventral to the epiblast, migrating in a cranial direction. They give rise to the prechordal plate, which in humans is considered to be endodermal. This plate grows to a maximum thickness of eight cell rows. Its cells later become both epithelial and mesenchymal. It is assumed that the epithelial cells of the prechordal plate generate the endodermal part of the oropharyngeal membrane and the epithelial lining of the foregut, amongst others, whereas mesenchymal cells give rise to a large part of the cephalic mesenchyme.
The prechordal plate is followed caudally by the prechordal mesenchyme and the notochordal process, from which the notochord arises. The notochordal process consists of both mesoderm and endoderm. The cranial part comes immediately after the prechordal mesenchyme. The middle part contains the notochordal canal, which can open ventrally and establish for just one day (E18) a direct connection (neurenteric canal) between the umbilical vesicle and the amniotic cavity. The caudal part forms an epithelial cell layer.
The notochordal process becomes a cell layer that continues laterally into the surrounding endoderm and, together with the endoderm, forms the roof of the umbilical vesicle. Once this takes place, the structure formed is called the notochordal plate. The notochordal plate then forms a longitudinal groove, being convex in shape at its dorsal end. The vertical edges of the groove move ventrally and medially to come together and form the rod-shaped notochord (chorda dorsalis), which detaches from the endoderm. The notochord is surrounded by a basal lamina. Its cranial end terminates at the prechordal mesenchyme.
It is widely accepted that the formation of axial structures and that of the three germ layers in humans is different from gastrulation in amphibians. Therefore, the term gastrulation should not be used for this developmental stage of the human embryo.