The sperm fertilizes the egg and forms a 2N zygote. Once fertilization is completed, a succession of rapid cell divisions ensues. During cleavage, the cells undergo the S (DNA synthesis) and M (mitosis) phases of the cell cycle but virtually skip the G1 and G2 phases, so little or no protein synthesis occurs. The embryo does not enlarge during this period of development. Cleavage parts the cytoplasm into many smaller cells called blastomeres, each with its own nucleus. The first five to seven divisions form the morula. …
