The right answer is blastocyst.
The blastocystic Stage of embryo development is characterized by the formation in the center of the morula (embryonic cell group) of a cavity isolated from the external environment: the blastocoele.
In embryonic development, this step precedes gastrulation.
Blastocysts are indeed composed of a hundred cells; they have the shape of a hollow sphere at the bottom of which is the mound of the "embryonic bud" from which the embryonic stem cells are derived.
