DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Coloured scanning electron micrograph (SEM) of a sperm (green) fertilising a human egg (brown). Women usually produce one large egg (ovum) for fertilisation, whereas men release some 300 million much smaller sperm (spermatozoa) which travel with whiplash movements of their tail through the uterus to reach the egg. A few hundred sperm survive this journey and encounter the egg in the fallopian tube. However, only one sperm succeeds in penetrating the egg's wall and fertilises the egg by fusing with its nucleus. When this occurs, the egg membrane forms a barrier to other sperm. Magnification: x3,500 at 6x7cm size.
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