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.