Intra- and extra-embryonic cavities

Formation of the mesoderm

The embryoblast differentiates to form the embryonic disc. The embryonic disk consists of two layers, the epiblast and the hypoblast. It then becomes elongated. At S6, epiblast cells in the caudal part of the embryonic disc migrate from medial and adjacent regions in a ventral direction and form the primitive streak. The cranial end of the primitive streak becomes the primitive node. At S7, the migration of epiblast cells increases considerably. The developmental potential of the migrated cells is restricted, and they become mesodermal and endodermal cells.
The cells originating from the primitive node move mainly in a cranial direction. The first of these migrating cells form the prechordal plate. In human embryos, this becomes a multilayered, rectangular structure with up to eight rows of cells, and seems to consist of both endodermal and mesodermal cells. It becomes integrated into the hypoblast layer. The epithelial cells of the prechordal plate give rise to the endoderm of the oropharyngeal membrane and to a portion of the endodermal lining of the foregut. In the region of the oropharyngeal membrane, ectoderm and endoderm remain in direct contact with no interleaving mesoderm. The next group of cells to migrate out from the primitive node give rise to the notochordal process that extends along the median plane up to the prechordal plate.
Cells originating from the primitive streak form the intra-embryonic mesoderm, which expands as a third layer between the epiblast and hypoblast. These cells also form the endoderm, which pushes the hypoblast sidewards and replaces it. At the caudal end of the primitive streak as well as in the area of the oropharyngeal membrane, a region will remain where ectoderm and endoderm are in direct contact with each other (cloacal membrane).