Blastula, or blastocyst, is the term used to describe an early stage in the development of an embryo. At this stage, the embryo consists of a hollow ball of cells enclosing a cavity called the blastocele. A fluid-filled cavity forms between one side of the inner cell mass and the outer cell mass, and the zona pellucida, a jellylike layer of cells covering a newly released egg, begins to disintegrate. The cells of the blastula divide as it floats free in the uterus for one or two days.