Proposed by Alan Turing in the 1952 paper, _The Chemical Basis of Morphogenesis_. This simple model consists of two morphogens (activating and inhibiting). The two morphogens diffuse at different rates-- the inhibiting generally diffuses faster. That leads to the activating morphogen being surrounded by the inhibiting, and that in turn to little islands surrounded by smooth, periodic regions of inhibiting morphogen. [[Reaction-diffussion model yields periodic patterns and successfully breaks the symmetry of a starting system.]]