DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Egg fertilisation. Coloured scanning electron micrograph (SEM) of a human spermatozoan (sperm) entering an egg (ovum) prior to fertilisation. The sperm is burying its head in the spongy surface of the zona pellucida (green). The long tail of the sperm was used to propel it to the egg. The zona pellucida is a thick protective layer around the egg which attracts sperm, as well as providing them with a site of attachment. The human female usually produces one egg each month, and only one of the millions of male sperm released may fertilise it. Magnification: x6,000 at 6x7cm size.
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