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Fertilisation. Light micrograph of a human egg cell during fertilisation. This egg contains two pronuclei (meeting at centre left), one from the egg and one from the sperm. When these fuse, the egg becomes a zygote containing the full 46 human chromosomes, 23 from each parent. Around the egg is the thick zona pellucida layer, surrounded by several sperm cells. This egg will soon begin its first cell division. Magnification unknown. (KEYSTONE/Science Photo Library/PASCAL GOETGHELUCK)