A process during early embryonic development in animals in which large-scale cell migration occurs and different cell types emerge to form the germ layers (endoderm, ectoderm, and sometimes mesoderm). The stage takes place just after the blastula stage and begins with an invagination (indenting) of the hollow ball of cells. During this process, the embryo is called a gastrula.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.