On the 4th or 5th day (stage 3) the embryoblast consists of a double-layered embryonic disk.
The outer cylindrical cells are known as epiblast (ectoblast) and the inner cubical cell layer is called the hypoblast (endoblast).
The amniotic cavity (Figs. 4 - 6) has developed into the epiblast, in that fluids have accumulated between the cells. The fluid that collects there splits the epiblast-cell into two layers. The cell layer at the embryonic pole differentiates itself into the(the cells of which are called amnioblasts). The amniotic membrane separates the amniotic cavity from the cytotrophoblast (stage 4).
The formation of the umbilical vesicle and of the chorion cavity (extra-embryonic coelom) is less well understood. Various theories exist. One says that the hypoblast cells from the edge of the embryonic disk migrate along the inner side of the blastocyst cavity and thereby flatten out (Fig. 6). From the 9th day a fine basal membrane-like layer outside this hypoblast layer is visible. It is called the exocoel-membrane or Heuser's membrane. Together they form the primary umbilical vesicle.
Also, the formation of the subsequent secondary umbilical vesicle is not yet fully clear. One theory points out that the secondary umbilical vesicle probably arises through the disintegration of the primary and restoration of the now secondary umbilical vesicle. Thereby the residues (remains) of the primary umbilical vesicle at the abembryonic pole decay and disappear (Fig. 12).
Simultaneously with the formation of the umbilical vesicle extra-embryonic mesoblast cells arise and cover the interior of the cytothrophoblast and the exterior of the umbilical vesicle as well as the amniotic cavity. The origin of these cells is still not yet entirely clarified. They could come from delaminated cytothrophoblast cells or from a proliferation zone at the caudal end of the embryonic disk (i.e., from the epiblast). Some authors postulate that they come from both the CT cells and a proliferation zone in the periphery of the embryonic disk. The extra-embryonic reticulum, which lies between these two layers of the extra-embryonic mesoblast (EEM), is dissolved and leaves a further cavity behind, the chorionic cavity (stage 6a).
On the 13th day the embryonic disk possesses dorsally an amniotic cavity and ventrally the chorionic cavity, which surround the umbilical vesicle outwardly. They seem to be hung only on a thick stalk from the mesoblast, the(stage 6b).