RICHARD RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY RICHARD RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Coloured light micrograph of a single sperm fertilising a human egg. The human female produces usually a single large egg or ovum (at right, granular). At upper centre, a single sperm is penetrating the egg's wall to fuse with the egg nucleus. When this happens, the egg membrane forms a barrier to other sperm. 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 succeed in fertilisation.
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