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Hydra, fruit flies, and stripy colonies of bacteria

In 1952, two years before his untimely death at the age of 41, the mathematician Alan Turing wrote an influential paper entitled "The Chemical Basis of Morphogenesis." The paper tackled the problem of how limbs and other structural patterns arise in plants and animals that begin life as undifferentiated blobs of cells.

Turing's mechanism relies on the competition between a slow-diffusing chemical—a morphogen—that activates a reaction and a fast-diffusing chemical that inhibits the reaction. Nudging the reaction-diffusion system into a metastable state yields stable stripes, spots, and other patterns.

Judging by his paper's abstract, Turing was inspired, in part, by Hydra, a genus of simple, water-dwelling animals whose body plan consists of a single sticky foot, a stem, and 1–12 thin, neurotoxin-charged tentacles. Although his mechanism presumes a continuous, two-dimensional system, its basic premise—that pattern development is controlled by the concentration-dependent diffusion and inhibition of signaling molecules—is observed in three-dimensional, multicellular systems, notably in biologists' favorite fly, Drosophila melanogaster.

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