The zygote undergoes cleavage, increasing the number of cells within the zona pellucida. After the 8-cell stage, mammalian embryos undergo what is called compactation, where the cells bind tightly to each other, forming a compact sphere. After compactation, the embryo is in the morula stage (16 cells). Cavitation ocurrs next, where the outermost layer of cells - the trophoblast - secrete water into the morula. As a consequence of this when the number of cells reaches 40 to 150, a central, fluid-filled cavity (blastocoel) has been formed. The zona pellucida begins to degenerate, allowing the embryo to increase its volume. This stage in the developing embryo, reached after four to six days, is the blastocyst (akin to the blastula stage), and lasts approximately until the implantation in the uterus. The blastocyst is characterized by a group of cells, called the inner cell mass (also called embryoblast) and the trophoblast (the outer cells).