·adj Of or pertaining to a gastrula.
II. Gastrula ·noun 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.