In animals, fertilization is the fusion of a sperm cell with an egg cell. The penetration of the egg cell by the chromosome-containing part of the sperm cell causes a reaction that prevents additional sperm cells from entering the egg. The egg and sperm each contribute half of the new organism's genetic material. A fertilized egg cell is known as a zygote. Following fertilization, the zygote undergoes continuous cell division that eventually produces a new multicellular organism.

Human fertilization in vivo (in the living body) occurs in oviducts (fallopian tubes) of the female reproductive tract, and takes place within hours following sexual intercourse. Only one of the approximately 300 million sperm released into a female's vagina during intercourse can fertilize the single female egg cell (ovum). The successful sperm cell must enter the uterus and swim up the fallopian...