A major consequence of CI is the delayed entry into mitosis of the male pronucleus. The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. The sperm contributes a pronucleus and a centriole to the egg. After plasmogamy, the microtubule plus ends continue to grow towards the opposite pronucleus. Meanwhile the female pronucleus is demethylated passively during consecutive cell divisions. The pronucleus was originally found during the first studies on meiosis. In mammals, the female pronucleus starts in the center of the egg before fertilization. There is asymmetry at this point&colon; the male pronucleus undergoes a quick and active demethylation.