n. - 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.