DON FAWCETT / SCIENCE PHOTO LIBRARY DON FAWCETT / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured Scanning Electron Micro- graph (SEM) of a human sperm penetrating an egg (ovum) during fertilisation. The head and tail of the sperm (blue) can be seen; the oval sperm head is at lower left. At lower right is the rounded surface of the egg (pink). The human female usually produces a single large egg, and a sperm must penetrate the egg's wall to fuse with the egg nucleus. When this happens, the egg membrane forms a barrier to any other sperm. The male releases some 300 million much smaller sperm which travel with whiplash movements of their tail through the uterus, to reach the egg. Fertilisation enables male and female chromosomes to combine.
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