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 Gastraea.