The answer for the above question is gastrulation.
Gastrulation means the formation of the gut, and also describes the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, divides all 3 embryo layers: endoderm, mesoderm and ectoderm. It is a phase early in the embryonic development of most animals during which the single layered blastula is reorganized into a trilaminar structure called the gastrula.
