DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Egg fertilisation. Coloured scanning electron micrograph (SEM) of sperm on the surface of a human egg (ovum) during fertilisation. The spongy surface of the zona pellucida, a layer surrounding the egg, is seen (yellow). Sperm (coloured green) are hair-like structures, each with a rounded head, neck, and a long tail. At centre, one sperm is penetrating the thick zona pellucida to reach the egg underneath. The zona pellucida protects the egg, and serves to attract the sperm and enables them to attach. The human female usually produces a single large egg, and only one of the millions of male sperm released may fertilise the egg. Magnification: x2,600 at 5x7cm size.
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