\Gas*tr[ae]"a\, n. [NL., from Gr. ?, ?, the stomach.]
(Biol.)
A primeval larval form; a double-walled sac from which,
according to the hypothesis of Haeckel, man and all other
animals, that in the first stages of their individual
evolution pass through a two-layered structural stage, or
gastrula form, must have descended. This idea constitutes the
Gastr[ae]a theory of Haeckel. See {Gastrula}.