Fertilisation. Conceptual computer artwork of human sperm swimming towards a bright light representing a female egg. Sperm (spermatozoa) are male sex cells formed in the testes. The head of each sperm contains genetic material that is transferred to the female egg during fertilisation. The flexible tail enables the sperm to propel themselves along the female reproductive tract. Only one sperm can successfully penetrate the egg and fertilise it by fusion of its genetic material with that of the egg. Each sperm contains 23 chromosomes, including an X or Y sex chromosome.