Deposited in the inhospitable conditions of the vagina, millions of sperm die before passing through the cervix. In the uterus and oviducts, millions more sperm die or become infertile, and millions choose the wrong oviduct or never find the oviducts at all. Nonetheless, contractions by the female genital tract do help the sperm in their journey toward the egg. In humans, the egg does not remain fertile longer than the time it takes for the egg to travel from the ovary through the upper third of the oviduct. Thus, human sperm must meet the egg there in order for fertilization to be possible. In humans, usually, only a three day period exists in which fertilization can occur. Fertilization, the fusion of the nuclei of sperm and ovum, occurs almost immediately after the sperm penetrates the membrane of the egg. Fertilization causes the membrane of the ovum to become impermeable to other sperm insuring only one sperm will penetrate the egg.
