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Sperm fertilising egg. Computer illustration of sperm cells (white) approaching a human egg (pink), before one of the sperm fertilises it. Women usually produce one large egg (ovum), whereas men release some 300 million much smaller sperm (spermatozoa) which travel with whiplash movements of their tails through the uterus to reach the egg. A few hundred sperm survive this journey and encounter the egg in the fallopian tube. Only one sperm can penetrate the egg's thick outer layer (zona pellucida) and fertilise it by fusing with its DNA (deoxyribonucleic acid) genetic material. When this occurs, the egg membrane forms a barrier to other sperm. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID GIFFORD)