Implantation of the blastocyst in the endometrium of the uterus is completed during the second week of the development. The cells of the inner cell mass (embryoblast) differentiate into two layers: the hypoblast layer, consisting of small cuboidal cells, and the epiblast layer, consisting of high columnar cells. The two-layered plate that will differentiate into the embryo is called the embryonic disc.
The epiblast forms the floor of the amniotic cavity and is peripherally continuous with a thin epithelial layer of the amnion. Flattened cells probably originating from the hypoblast, form a exocoelomic membrane (Hauser's membrane). This membrane and the hypoblast form the lining of the exocoelomic cavity (primitive yolk sac).