False-colour scanning electron micrograph (SEM) of a group of human spermatozoa, the motile, male sex cells, showing their division into head and tail sections. The sperm's tail enables it to swim, an essential action in reaching the female ovum. The mature male sex cell (male gamete) is located in the arrow-like head, the tip of which is capped by the acrosome, which releases enzymes that assist in the penetration of the egg during fertilisation. Only a single sperm succeeds in fertilisation; rapid chemical changes cause the outer membrane surrounding the ovum to thicken and form a barrier to competing sperm. Magnification: x1260 at 35mm size, x2520 at 6x7cm size.