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Fertilisation. Light micrograph of sperm cells (spermatozoa) attempting to penetrate a human egg (ovum, centre). Each sperm has a rounded head and a long tail with which it swims. Women usually release one egg per month, whereas men release millions of sperm in each ejaculation. Only one of these sperm can penetrate the egg's thick outer layer, the zona pellucida, and fertilise it. Fertilisation occurs when the sperm's genetic material (deoxyribonucleic acid, DNA) fuses with the egg's DNA. When this occurs the egg forms a barrier to other sperm. Magnification: x625 when printed at 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/AJ PHOTO)