A hollow ball of cells with one part of the ball pushed into the interior blastocoele (called an invagination). This early stage of animal embryonic development forms just after the blastula stage. By the end of the gastrulation process, the different embryonic germ layers (endoderm, ectoderm, and sometimes mesoderm) can clearly be identified.

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