Alternation of generations. Plants go through two stages in their life cycle, called alternation of generations. One is the diploid stage, in which cells undergoing meiosis produce haploid reproductive cells called spores. During the haploid stage, the spores develop into gametophytes (or gamete-producing plants) that produce haploid gametes (eggs and sperm) by mitosis. The gametes unite to produce a diploid zygote that grows into a sporophyte (spore-producing plant), thus completing the cycle.
Fertilization is the process by which a sperm combines with an oocyte, or egg cell, to produce a fertilized zygote. The sperm released during ejaculation must first swim through the vagina and uterus and into the fallopian tubes where they may find an oocyte. After encountering the oocyte, sperm next have to penetrate the outer corona radiata and zona pellucida layers of the oocyte. Sperm contain enzymes in the acrosome region of the head that allow them to penetrate these layers. After penetrating the interior of the oocyte, the nuclei of these haploid cells fuse to form a diploid cell known as a zygote. The zygote cell begins cell division to form an embryo.
Each sperm produced by the testes takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat that keeps the testes and sperm at the correct temperature. It may be surprising that the testes should lie in such a vulnerable place outside the body, but it is too hot for them inside. Spermatogenesis requires a temperature that is three to five degrees Fahrenheit below body temperature. If it becomes too cool on the outside, the cremaster muscle will contract to bring the testes closer the body for warmth.