The epiblast migrates away from the trophoblast downwards, forming the amniotic cavity; the hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining.
The epiblast migrates away from the trophoblast downwards, forming the amniotic cavity, the lining of which is formed from "amnioblasts" developed from the epiblast. The hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining.