It is shown that the formation of cellular structure in a flame is conditioned by diffusion and heat conduction effects and is independent of the hydrodynamics of the perturbed flame. It is proved that cellular flames are formed only when a sufficiently light reactant of the combustible mixture is present in a low concentration. If there is an excess of the light reactant, a plane flame front is absolutely stable (in the framework of the diffusional-thermal model) or shows a noncellular periodic structure (hydrodynamic model)