Schematic drawing showing the difference between the morphogen gradient model and Turing model. (A) A morphogen molecule produced at one end of an embryo forms a gradient by diffusion. Cells “know” their position from the concentration of the molecule. The gradient is totally dependent on the prepattern of the morphogen source (boundary condition). (B) Adding a second morphogen produces a relatively complex pattern; but with no interactions between the morphogens, the system is not selfregulating. (C) With addition of the interactions between the morphogens, the system becomes selfregulating and can form a variety of patterns independent of the prepattern. Picture and caption taken from Kondo and Miura, 2010.