Mammalian brains vary greatly in size and expanse of neocortex. Yet, regardless of the extent, much of the cortex consists of orderly representations of receptor surfaces. Many of these representations closely reflect the order of their receptor sheet. The fidelity of the match can be so exact that discontinuities in the receptor sheet, such as the optic disc of the retina or the separations between fingers, are reflected in the representations. If parts of the receptor surface are duplicated or missing in development, representations are appropriately altered. Such isomorphisms suggest that the receptor sheet instructs the central representations to influence the course of their development. Representations may be fractured into a mosaic of small partial maps, possibly as a result of competing factors in development. Parts of receptor surfaces can achieve proportionately more than their share of a sensory representation. The congruence of borders between representations suggests the transfer of instructions across borders. Neural activity patterns are a likely source of developmental information in all these instances.