an embryonic form having its origin in the invagination or pushing in of the wall of the planula or blastula (the blastosphere) on one side, thus giving rise to a double-walled sac, with one opening or mouth (the blastopore) which leads into the cavity (the archenteron) lined by the inner wall (the hypoblast). see illust. under invagination. in a more general sense, an ideal stage in embryonic development. see gastraea.-of or pertaining to a gastrula.