The outer cell layer of the early gastrula is the ectoderm; the inner layer is the endoderm. The next major change is the development of the middle cell layer, the mesoderm. Mesoderm develops as pouches from the endoderm. The embryo at the late gastrula stage has three cell layers and the head-tail axis is well established. There is a flattened region on the dorsal surface of the embryo; this is the beginning of the nerve cord.
