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Embryonic hand. Coloured scanning electron micrograph (SEM) of the hand of an embryo during early development. The hands develop from two limb buds, visible at the beginning of the fifth week of pregnancy. By the sixth week the limb buds have flattened, forming two primitive hands known as hand plates. The fingers are formed later, when programmed tissue death separates the hand plates into five parts. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)