A morphogenetic process involving the inward folding of an unbroken layer of cells into the central cavity of the blastula during the initial stage of gastrulation. It results in a considerable re-organization of the embryo from a spherical balls (i.e., blastula) into a multilayered organism, with differentiated germ layers (viz: entoderm. mesoderm, ectoderm)
See Bastula, Ectoderm, Embryo, Entoderm (or endoderm), Gastrulation, Germinal (or germ) layers, Mesoderm, Morphogenesis