The process of gastrulation occurs during the early embryogenesis of vertebrates, immediately following the phase of cleavage and when the embryo is at the blastula stage. Gastrulation results in the formation of the primitive gut or archenteron. With the initial formation of the archenteron, the embryo reaches the gastrula stage. As more broadly defined, gastrulation is the phase of development in which the three primary germ layers are established: the inner endoderm (i.e., the layer closest to the yolk and the layer that forms the archenteron), the outer ectoderm (i.e., the layer on the surface of the egg), and the mesoderm (i.e., the layer interposed between the endoderm and ectoderm).
