part of part of immediately preceded by immediately preceded by occurrent very early blastula stage stage 8 early blastula stage B2 EB The embryo is composed of about 100-200 cells and is surrounded by a fertilization envelope. Most blastomeres have completed at least one additional mitotic division since the 60- or 64-cell stage. All cells are approximately the same size and there are no obvious morphological indications of the primary or secondary embryonic axes. All cells have begun to acquire an apical-basal polarity and the embryo is beginning to transform into a hollow sphere with a monolayered epithelial wall and a fluid-filled cavity (the blastocoel) in the center. ecao_developmental_stage