(embryology) The process in which the embryo develops into a gastrula following blastulation during the early embryonic development of animals
An animal embryo undergoes a series of developmental stages. Soon after fertilization, the embryo divides mitotically until it becomes a blastula. The embryonic cells then migrate in complex and coordinated series of cellular movements. One of which is by invagination. The end result is the formation of a gastrula, with three germ layers: ectoderm, mesoderm, and endoderm.