CLAUS LUNAU / SCIENCE PHOTO LIBRARY CLAUS LUNAU / SCIENCE PHOTO LIBRARY
Sperm cell fertilising an ovum, illustration. Each sperm (spermatozoan, blue) has a rounded head and a tail which it uses to both swim and burrow into the egg (yellow). 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. The sperm cell releases chemicals (pink and then green) to penetrate the ovum's outer layers. Fertilisation occurs when the genetic material (DNA, deoxyribonucleic acid, red) in the sperm's head fuses with the egg's DNA to form a zygote. The egg then forms a barrier to other sperm.
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