in the formation of polymeric layers, especially anisotropic polymeric layers, on a solid substrate on which the layer is formed by cross-linking of an organic substance which can be cross-linked, the cross-linking is performed in a manner such that a concentration gradient of at least one of the components controlling the cross-linking reaction is formed in the boundary layer between the substrate to be coated and the surrounding solution, which concentration gradient is adjusted so that cross-linking can take place in the boundary layer but not in the surrounding solution. The concentration gradient is preferably achieved by impregnating the substrate with at least one of the components which control the cross-linking reaction and which tends to diffuse into the solution on contact therewith.