As each cartilage condensation (blue, Sox9) forms, it becomes repressive to the formation of nearby condensations. The diffusion and strength of repression give a "wavelength" shown in purple, outside the wave, condensations can form (X-axis is proximal-distal axis and Y-axis is strength of repression). The yellow region is a region of instability (high concentration of FGFs from the Apical Ectodermal Ridge) where condensations cannot form. As the limb grows larger, there is room for more condensations with the same wavelength.
Figure originally published in Evolutionary Developmental Biology
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limb, FGF, SOX9, turing, AER, apical ectodermal ridge, autopod, evodevo, evolution, development, digits