At day 9-10 of embryonic development, cells from the hypoblast begin to migrate to the aembryonic pole, forming a layer of cells just beneath the cytotrophoblast, called Heuser's Membrane. It surrounds the exocoelomic cavity (primitive yolk sac), i.e. it lines the inner surface of the cytotrophoblast. At this point, the exocoelomic cavity replaces the blastocyst cavity.
At days 11 to 12, there is further delineation of the trophoblastic cells giving rise to a layer of loosely arranged cells that inserts between Heuser’s membrane and both syncytiotrophoblast and cytotrophoblast.