 (These equations follow Keller’s 2002 presentation, which is more streamlined than Turing’s original.) One morphogen is an activator, the other an inhibitor. Both are made and destroyed at constant rates. Initially, the tissue is homogeneous, lacking any spatial organization. Crucially, for stable patterns to emerge X and Y must diffuse at different rates, the inhibitor having the larger diffusion coefficient. Under these parameter values, random fluctuations in the system lead to instability resulting in pattern-formation. That is, from an initially stable, homogeneous system of cells/tissue, interaction between two diffusing chemicals (morphogens) leads to emergence of a stable chemical pattern. Turing assumes that steady-state chemical patterns determine patterns at the cell/tissue level (through mechanisms wholly unspecified). Given that assumption, the model accounts for pattern formation in a group of cells or a tissue.