A blastocoel (alt. spelling blastocoele, blastocele), also termed the blastocyst cavity (or cleavage or segmentation cavity) is the name given to the fluid-filled cavity of the blastula (blastocyst) that results from cleavage of the oocyte (ovum) after fertilization. It forms during embryogenesis, as what has been termed a "Third Stage" after the single-celled fertilized oocyte (zygote, ovum) has divided into 16-32 cells, via the process of mitosis. It can be described as the first cell cavity formed as the embryo enlarges, the essential precursor for the differentiated, topologically distinct, gastrula. The adjectival form of blastocoel is blastocoelic.