Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

gastrula
     n : 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
     [also: {gastrulae} (pl)]

gastrulae
     See {gastrula}