Gastrulation is the formation of a gastrula from a blastula. Gastrulation is a phase in the embryonic development of most animals. It consists of a complex and coordinated series of cellular movements which occurs at the end of cleavage. The details of these movements vary among species, but usually result in the formation of an embryonic stage called a 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.