blastula vs. gastrula

the early developmental stage of an animal, following the morula stage and consisting of a single, spherical layer of cells enclosing a hollow, central cavity.

a metazoan embryo in an early state of germ layer formation following the blastula stage, consisting of a cuplike body of two layers of cells, the ectoderm and endoderm, enclosing a central cavity, or archenteron, that opens to the outside by the blastopore: in most animals progressing to the formation of a third cell layer, the mesoderm.