DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Sperm fertilising egg. Light micrograph of a human egg with sperm at its surface attempting to penetrate and fertilise it. The large size of the rounded egg is seen in relation to the tiny sperm at upper frame. Around the egg is a thick zona pellucida layer. Women usually produce one egg (ovum) for fertilisation, whereas men release some 300 million sperm (spermatozoa) which travel with whiplash movements of their tail through the uterus to reach the egg. 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: x260 at 5x7cm size.
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