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 Gastræa.
Embryo that is in the process of regionalizing into three germ layers (ectoderm, mesoderm and endoderm).
the embryonic stage of an animal that has cells differentiated into germ layers. Sequentially, It follows the blastula stage An early gastrula. The cells at the vegetal hemisphere have begun to invaginate into the blastocoel, forming the archenteron. (Photo: Dr. Anna E. Ross, Christian Brothers University, TN)