D. PHILLIPS / SCIENCE PHOTO LIBRARY D. PHILLIPS / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Coloured scanning electron micrograph of a single sperm fertilising a human egg. The human female produces usually a single large egg or ovum (at lower frame) for fertilisation, whereas the male releases some 300 million much smaller sperm (spermatozoa) which travel with whiplash movements of their tail through the uterus, to reach the egg. Only a few hundred sperm survive this journey and encounter the egg in the fallopian tube, and only one sperm can actually fertilise the egg. The sperm succeeds in penetrating the egg's wall to fuse with the egg nucleus. When this happens, the egg membrane forms a barrier to other sperm. Magnification: unknown.
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