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Arbacia eggs 32 cells, 400X w.m.
In echinoderms, the first sign of gastrulation (formation of the three germ layers) is the flattening of the blastula at the vegetal pole. This flattened region then gradually folds inward, converting the spherical blastula into a cup-shaped gastrula. During gastrulation, the blastocoel is displaced and a new cavity, the archenteron, is formed. The opening into the archenteron is the blastopore and develops into the anus in all deuterostomes. By the late gastrula stage all three germ layers are visible with ectoderm forming the outside surface, endoderm lining the archenteron, and enterocoelic mesodermal pouches located on either side of the archenteron.