FERTILIZATION: SECOND TWO EVENTS
Release of calcium ions into the egg cytoplasm also triggers the activation program, a series of metabolic changes within the egg. Aerobic respiration increases, various maternal enzymes and other proteins become active, and within a few minutes after sperm entry, a burst of protein synthesis occurs. In addition, the egg nucleus is stimulated to finish meiosis.

After the sperm nucleus enters the egg, it is thought to be conducted toward the egg nucleus by a structure of microtubules within the egg. The sperm nucleus swells, forming the male pronucleus. The nucleus formed in the egg at the end of meiosis becomes the female pronucleus. The haploid male and female pronuclei then fuse to form the diploid nucleus of the zygote. DNA synthesis occurs as the first step toward cell division.