ANIMATED HEALTHCARE LTD / SCIENCE PHOTO LIBRARY ANIMATED HEALTHCARE LTD / SCIENCE PHOTO LIBRARY
Fertilisation. Computer artwork of sperm cells attempting to penetrate a human egg (centre). Each sperm (spermatozoan) has a rounded head and a long tail which it uses to both swim and burrow into the egg. Women usually release one egg (ovum) per month, whereas men release millions of sperm in each ejaculation. Only one of these sperm will penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when the sperm's genetic material (deoxyribonucleic acid, DNA) fuses with the egg's DNA forming a zygote. When this occurs the egg forms a barrier to other sperm.
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