False-colour scanning electron micrograph (SEM) of human sperm (pale brown) surrounding & attached to a human egg (pale pink). In fertilisation, only one sperm succeeds in penetrating the egg's wall to fuse with the nucleus at its centre. As soon as one sperm has successfully broken through, rapid chemical changes cause the outer membrane of the egg to thicken and form a barrier to competing sperm. Magnification: x400 at 35mm, x800 at 6x7cm size. Reference: MICROCOSMOS, figure 2.7, page 17.