SUSUMU NISHINAGA / SCIENCE PHOTO LIBRARY SUSUMU NISHINAGA / SCIENCE PHOTO LIBRARY
Sperm cells. Coloured scanning electron micrograph (SEM) of sperm cells, or spermatozoa. These tiny male sex cells are produced in the testes, and are responsible for fertilisation of the female egg (ovum). Each sperm has a rounded head that contains the male hereditary material (DNA) and a long tail that beats to propel the sperm. Some 300 million sperm are released by the male during ejaculation, while the female usually produces one large egg at a time. Only one sperm cell can fertilise the egg. Magnification unknown.
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