At what stage of animal development do cells first move from the surface of the embryo into the interior, resulting in a two- or three-layered embryo?

A.	zygote formation
B.	blastulation
C.	neurulation
D.	gastrulation Gastrulation begins when the wall of the single-layered blastula begins to invaginate. This invagination, and the subsequent rearrangement of cells creates a multi-layered embryo.
E.	metamorphosis