Review of the development during the 2nd embryonic week
In the bilaminar primordium of the embryo (hypoblast or primary endoderm and epiblast) one recognizes in the epithelium of the epiblast a fluid-filled space, the first primordium of the amniotic cavity.
Ventrally, the roof of the still incompletely uncovered primary umbilical vesicle (previously the blastocyst cavity) is formed by the hypoblast.
Schematically, amniotic cavity and primary umbilical vesicle together form two hemispheres with two layers (epi- and hypoblast) lying close to one another, thus representing the first embryonic primordium.
However, only the epiblast is responsible for forming the embryo. The hypoblast develops into a part of the extraembryonic appendages