Blastocyst Formation At the next stage of development (morula), which produces an embryo with about $64\; cells$, a cavity is formed with in the cell mass. This cavity is called blastocyst cavity (blastocoel) and the embryo is termed as blastocyst. Blastocyst composed of an outer envelops of cells the trophoblast or trophoectoderm and inner mass cell (embryoblast). The side of the blastocyst to which inner mass cell is attached is called embryonic pole (animal pole), while opposite side is the abembryonic pole