has_part has part has part ectoderm formation endoderm formation mesoderm formation gastrulation A complex and coordinated series of cellular movements that occurs at the end of cleavage during embryonic development of most animals. The details of gastrulation vary from species to species, but usually result in the formation of the three primary germ layers, ectoderm, mesoderm and endoderm. GO:0007369 Wikipedia:Gastrulation biological_process embryonic morphogenesis