D. PHILLIPS / SCIENCE PHOTO LIBRARY D. PHILLIPS / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured scanning electron micro- graph (SEM) of a single sperm fertilising a human egg. Whereas the human female produces usually a single large egg for fertilisation, the human male releases some 300 million much smaller sperm which must travel with whiplash movements of their tail, through the uterus to reach the egg. Fertilisation occurs in the fallopian tube. Only a few hundred sperm survive the journey and encounter the egg, and only one sperm can actually fertilise the egg. The sperm penetrates the egg's wall to fuse with the egg nucleus. As soon as this happens, the egg membrane forms a barrier to other sperm. Magnification: x690 at 5x7cm size.
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