||Gas"tru*la (?), n.; pl. Gastrulæ (#) [NL., dim. fr. Gr. &?; the stomach.] (Biol.) 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 Gastræa. -- a. Of or pertaining to a gastrula.