A blastocoel(e) or blastocele (also called blastocyst cavity, cleavage cavity or segmentation cavity) is the fluid-filled central region of a blastula and mammalian blastocyst. A blastocoele forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis. The adjectival of “blastocoel(e)” is blastocoelic. A blastocoel can be described as the first cell cavity formed as the embryo enlarges. It is essential for later gastrulation.