We present a method for setting up topographically ordered mappings between two sheets of nerve cells. A set of chemical markers that express the neighborhood relationships within the presynaptic sheet is induced by the fibers into the postsynaptic sheet. The markers are used to guide the fibers to their terminal sites. A case for which this idea may be relevant is the retinotectal projection; our model exhibits types of plasticity found experimentally. The fact that the postsynaptic markers remain after removal of the projecting fibers suggests an important difference between development and regeneration. This paper concentrates on explaining the basic idea, and in addition presents a set of preliminary computer simulations.