During embryonic development of most animals, a complex and coordinated series of cellular movements occurs at the end of cleavage. The details of these movements vary among species, but usually result in the formation of of an embryonic stage termed the gastrula. The gastrula has two primary germ layers, the ectoderm and endoderm in diploblastic animals, and three primary germ layers with the development of the mesoderm in triploblastic animals.