An embryonic form having its origin in the invagination orpushing in of the wall of the planula or blastula (the blastosphere)on one side, thus giving rise to a double-walled sac, with oneopening or mouth (the blastopore) which leads into the cavity (thearchenteron) lined by the inner wall (the hypoblast). See Illust.under Invagination. In a more general sense, an ideal stage inembryonic development. See Gastræa.-- a.

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