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.
The mesenchymal lineage of cells is mostly innate in function while the hematopoietic is mostly adaptive. (though there is, of course, overlap in function).
