layers. In contact with the blastocoel, the lower layer is called the primitive endoderm, and it is homologous to the chick embryo hypoblast. While hypoblast cells delaminate ventrally, away from the embryonic pole, to line the blastocoele the remaining cells of the inner cell mass, situated between the hypoblast and the polar trophoblast, become the epiblast.

In the mouse primordial germ cells