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 …showed first 75 words of 395 total…

…showed last 75 words of 395 total…just above the archenteron. Signals sent from the notochord to the ectoderm above it cause that region of ectoderm to become the neural plate. The neural plate soon curves inward, rolling itself into the neural tube which runs along the anterior-posterior axis of the embryo. The neural tube will become the central nervous system. A band of cells called the neural crest develops along the border where the neural tube pinches off from the ectoderm.