Stage of embryonic development, at / near the end of cleavage, preceding gastrulation; generally consisting of a hollow ball of cells ( coeloblastula ); if no blastoceol is present it is termed stereoblastulae (arise from isolecithal and moderately telolecithal ova); in meroblastic cleavage (only the upper part of the zygote is divided ), the blastula consists of a disc of cells lying on top of the yolk mass; the blastocoel is reduced to the space separating the cells from the yolk mass