ANIMATED HEALTHCARE LTD / SCIENCE PHOTO LIBRARY ANIMATED HEALTHCARE LTD / SCIENCE PHOTO LIBRARY
Fertilization. Computer artwork of human sperm (spermatozoa) at the surface of an egg (ovum). Only one sperm can penetrate the egg's thick outer layer (zona pellucida) and fertilize it. Fertilization occurs when DNA (deoxyribonucleic acid) released by the sperm fuses with DNA contained in the egg. The 23rd chromosome in a sperm cell determines the sex of the foetus, as it is either an X (female) or Y (male). Women produce a single egg at a time, whilst men release around 500 million sperm. The sperm use their flexible tails to travel through the female reproductive tract. Only a few thousand sperm reach the egg, which is located in one of the Fallopian tubes.
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