Long axon connections among different regions in the nervous system are established early in development. These connections mediate essential information transfer among specialized cerebral centers to form integrated functional systems. For example, the retina of the eye connects with multiple visual centers in the brain, including the lateral geniculate (visual) nucleus of the thalamus. The lateral geniculate nucleus is connected to the occipital (visual) cortex. The retina-lateral geniculate-visual cortex pathway mediates certain visual functions. Axons that connect retinal ganglion cells to the brain develop in three distinct phases: 1) elongation, 2) collateralization, and 3) arborization. First, unbranched axon trunks extend from the bodies of retinal ganglion cells and traverse all of their potential targets without forming connections with them. Second, the axon trunks sprout "collateral" branches in multiple targets simultaneously. Many of these collaterals are "exuberant" in the sense that they are formed within visual and nonvisual brain nuclei from which they later withdraw. Third, some of the collaterals of each axon grow and branch repeatedly to form mature terminal arbors, whereas other collaterals are eliminated. These processes are controlled by multiple mechanisms, including chemical signals from the target neurons and the electrical activity of neurons. Perturbations of these modulatory influences can alter the mature pattern of neuronal connections.