An embryonic form having its origin in the invagination or pushing in of the wall of the planula or blastula on one side, thus giving rise to a double- walled sac, with one opening or mouth which leads into the cavity lined by the inner wall.
Of or pertaining to a gastrula. double-walled stage of the embryo resulting from invagination of the blastula; the outer layer of cells is the ectoderm and the inner layer differentiates into the mesoderm and endoderm.