Color enhanced scanning electron micrograph (SEM) of sperm on the surface of a human egg (ovum) during fertilization. The egg has a furry surface which attracts sperm. The sperm's tail enables it to swim, and reach the female ovum. The head is capped by the acrosome that releases enzymes that assist in the penetration of the egg during fertilization. Only a single sperm succeeds in fertilization; rapid chemical changes thicken the outer membrane surrounding the ovum and form a barrier to competing sperm. Once fertilized, the egg begins the process of cell division.