Gastrula \Gas"tru*la\, n.; pl. Gastrul[ae][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 Gastr[ae]a. -- a. Of or pertaining to a
   gastrula.
   [1913 Webster]