False-colour scanning electron micrograph of the egg of a sea urchin surrounded by sperm. In the process of fertilisation, one sperm succeeds in entering the egg and its nucleus fuses with the egg's nucleus. The micrograph illustrates the vast size differential between the female germ cell, the egg, and the male germ cells, the sperm. Sea urchin eggs and sperm are widely used in fertilisation research because they are very similar to human ones. Original is BW print P649/19M.