AbstractThe development of the corpus callosum (CC) and the anterior commissure (CA) is well known in a wide variety of species. No study, however, has described the development of the commissure of the superior colliculus (CSC) from embryonic state to adulthood in mammals. In this study, by using the lipophylic tracer DiI, we investigated the ontogeny of this mesencephalic commissure in the hamster at various ages. The development of axonal terminals, growth cone morphologies, and axons branching were described for the superior colliculus (SC) contralateral to the tracer injection. The first CSC axons cross the midline at embryonic day 11 (E‐11) and grow further into the intermediate layers of the contralateral SC between E‐12 and E‐14. There is little axon growth therein between E‐14 and the day of birth (P‐0). Growth cones at the tip of these axons adopt complex morphologies at E‐12 and progressively simplify until P‐0. Pioneer axons are clearly visible between E‐14 and P‐1. These are followed by other axons progressively more numerous between P‐0 and P‐5. Axons do not show any branching until P‐2. Between P‐3 and P‐9, the axons progressively arborize in the intermediate layers. Some axons reach the superficial layers at P‐5, and they become more numerous around P‐11, and only a few axons remain therein by P‐21. Myelinated axons appear at P11 and are very dense at P‐21. Our results indicate that the CSC follows developmental schemes similar to those of the CC and the AC but that initial axon midline crossing occurs earlier. J. Comp. Neurol. 494:887–902, 2006. © 2005 Wiley‐Liss, Inc.