‖n. [ NL., from Gr. &unr_;, &unr_;, 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 Gastræa theory of Haeckel. See Gastrula. [ 1913 Webster ]