Reactions controlled by diffusion create self-organizing periodic patterns in chemical systems and play a key role in biological pattern formation through diffusion-generated morphogen gradients. To better understand the organizing ability of diffusion-controlled assembly, we investigated the formation of bilaterally symmetric morphologies by simple inorganic precipitation reactions in neutral agarose gel. Our results reveal that bilaterally symmetric 'face-like' deposits of insoluble products reproducibly form in gel, by reactions governed by the properties and concentrations of soluble reactants placed symmetrically relative to a midline and by the temporal and spatial distributions arising by diffusion from initial times and positions of reactant placement. (C) 2013 Published by Elsevier B.V.