A stepwise morphogenetic program of cell division and cell fate determination generates the precise neuronal architecture of the visual centers of the Drosophila brain. Here, we show that the assembly of the target structure for ingrowing retinal axons involves cell-cell interactions mediated by the secreted product of the wingless (wg) gene. wg, expressed in two symmetrical domains of the developing brain, is required to induce and maintain the expression of the secreted decapentaplegic (dpp) gene product in adjacent domains. wg and dpp function are required for target field neurons to adopt their proper fates and to send axons into the developing target structure. These observations implicate a cascade of diffusible signaling molecules in patterning the visual centers of the Drosophila brain.