<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Gastrulation is a phase early in the embryonic development of most animals, during which the single - layered blastula is reorganized into a multilayered structure known as the gastrula . Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body (e.g. dorsal - ventral, anterior - posterior), and internalized one or more cell types including the prospective gut . </P> <P> In triploblastic organisms the gastrula is trilaminar ("three - layered"). These three germ layers are known as the ectoderm, mesoderm, and endoderm . In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm . The two layers are also sometimes referred to as the hypoblast and epiblast . </P> <P> Gastrulation takes place after cleavage and the formation of the blastula . Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers . Each layer gives rise to specific tissues and organs in the developing embryo . The ectoderm gives rise to epidermis, the nervous system, and to the neural crest in vertebrates . The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas . The mesoderm gives rise to many cell types such as muscle, bone, and connective tissue . In vertebrates, mesoderm derivatives include the notochord, the heart, blood and blood vessels, the cartilage of the ribs and vertebrae, and the dermis . Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme . </P>

During embryonic development in the eumetazoa gastrulation results in