Gastrulation means

Gastrulation is a process by which blastocyst transforms into three layered (germ layers) structure called gastrula. These three germ layers are inner endoderm, middle mesoderm and outermost ectoderm. These three layers give rise to all tissues and organs of the developing foetus. Conversion of morula into blastula is called blastulation. In this stage, inner cell mass is found inside the blastula. Organogenesis occur after gastrula stage.