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:
In 1676 the Englishman Robert Hooke published, at the end of a book on helioscopes, instruments for observing the Sun, a list of discoveries he did not yet want to reveal. One of them read: $$\textit{ceiiinosssttuv}$$It was an anagram: the letters of a Latin sentence, shuffled and put in alphabetical order. It was a way of recording priority for a discovery without handing it to rivals. Two years later, in 1678, Hooke revealed the sentence: ut tensio, sic vis, “as the extension, so the force”. The more a spring is stretched, the harder it pulls back. Today we write it in four signs: