Cleavage itself is the first stage in blastulation, the process of forming the blastocyst. The dividing cells which are termed blastomeres (blastos Greek for sprout) are still enclosed within the strong membrane of glycoproteins (termed the zona pellucida) of the ovum, which the successful spermatozoon managed to penetrate. The zygote (which is large compared to any other cell) undergoes further cleavage, increasing the number of cells without any increase in the size of the initial zygote. This means that the proportion of nuclear genetic material is greater than that of the cytoplasm in each cell. When eight blastomeres have formed they are undifferentiated and aggregated into a sphere, and when the cells number about sixteen or thirty-two the solid sphere of cells is termed a morula. At this stage the cells start to bind firmly together in a process called compaction, and cleavage continues as cellular differentiation begins. Cells differentiate into an outer layer of cells, the trophoblasts, and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable, and are still enclosed within the zona pellucida. This compaction serves to make the structure watertight since the cells will later secrete fluid. The inner mass of cells differentiate to become embryoblasts and polarise at one end. They close together and form gap junctions in order to facilitate cellular communication. This polarisation leaves a cavity, the blastocoel in which is now termed the blastocyst. (In animals other than mammals, this is called the blastula). The trophoblasts secrete fluid into the blastocoel. By this time the size of the blastocyst has increased which makes it 'hatch' through the zona pellucida which then disintegrates