from
The Collaborative International Dictionary of English v.0.48
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]