Thumbnail: The process of gastrulation. Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers--the ectoderm, the mesoderm, and the endoderm. Some of the ectoderm cells from the blastula collapse inward and form the endoderm. The blastospore is the hole created in this action. Whether this blastospore develops into a mouth or an anus determines whether the organism is a protostome or a dueterostome. This diagram is color coded. Ectoderm, blue. Endoderm, green. Blastocoel (the yolk sack), yellow. Archenteron (the gut), purple. Image used with permission (Public Domain; Abigail Pyne).