Early cleavage through gastrulation in the ascidian (sea-squirt) Boltenia villosa. The embryo is surrounded by a chorion and follicle cell layer which it co-habits with many test cells; the movie begins at the 2-cell stage and ends just after gastrulation. The embryo is viewed from the vegetal pole; darker gray pigment (orange in true color) marks the posterior and the muscle lineage. Note how the endoderm cells pass through the focal plane together as they invaginate. 360x time/water-immersion 25x/DIC (George von Dassow)