Micrograph of a sperm cell. Spermatozoa are the highly motile male sex cell. The sperm's tail enables it to swim, an essential action in reaching the female ovum. The 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.