protogaster

from The Century Dictionary.

• noun In embryology, the central cavity of a gastrula; the primitive intestinal cavity of a two-layered germ; the hollow of the archenteron of a germ-cup, inclosed by the hypoblastic blastodermic membrane or endoderm, and communicating with the exterior by the protostoma or are hæostoma, which is the orifice of invagination of the antecedent blastula.