An embryo starts as a ball of almost identical cells. Even so, it ends up with five fingers on each hand, stripes on a zebra and spots on a leopard. Where does form come from, if at the start everything is uniform? In 1952, in the last major paper he published, “The chemical basis of morphogenesis”, Turing proposed a mathematical answer that biologists took decades to confirm. Two substances, two speeds The idea is to imagine two chemical substances, which Turing called morphogens, spreading through a tissue. Today they are usually described like this: