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.
. In a more general sense, an ideal stage in embryonic development. See Gastræa
. [1913 Webster