False-colour transmission electron micrograph (TEM) of an egg (ovum) during fertilisation, showing a single human spermatozoon that has just penetrated the zona pellucida, the thick membrane (top) that surrounds the oocyte, the female sex cell. The dart-shaped head of the sperm is resting in the space between the zona and ooocyte (bottom). The sperm's head houses the nucleus which contains the 23 chromosomes of the male sex cell. Following the successful penetration of the oocyte by a single sperm, rapid chemical changes in the membrane surrounding the oocyte cause it to thicken and prevent entry of competing sperm. Magnification: x1000 at 35mm size.