DR TONY BRAIN / SCIENCE PHOTO LIBRARY DR TONY BRAIN / SCIENCE PHOTO LIBRARY
Coloured Scanning Electron Micrograph (SEM) of mature human sperm colour-coded to symbolise those with an X (male) or Y (female) sex chromosome. X chromosome (pink) sperm produce a female foetus after fertilisation; Y chromosome (blue) sperm produce a male foetus. Since the egg always contains an X chromosome, it is the sperm that determines sex of the foetus: male (XY), female (XX). Sperm are the motile male sex cells produced in the testis. Each sperm has a head containing the genetic material, a neck, and a tail that enables it to swim. Millions of sperm are produced by the male, but only one can ferti- lize the egg. Magnification x1100 at 6x4.5cm size.
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