Dictionary.net

The mammalian embryo in the post-morula stage in which a fluid-filled cavity, enclosed primarily by trophoblast, contains an inner cell mass which becomes the embryonic disc.

A hollow globe of cells, with wall usually one layer thick.

The blastosphere.

The fecundated ovum in the stage immediately succeeding that of the morula, when it is a hollow sphere filled with liquid or a gelatinous substance, and its walls consist of a single layer of cells. [Gr.]