Developing axons of the corpus callosum of mice are guided across the cerebral midline by a slinglike glial structure that forms transiently between the hemispheres. If the "sling" is cut at precallosal stages, the would-be callosal fibers whirl into paired neuromas adjacent to the longitudinal cerebral fissure. In experiments on such surgically acallosal animals, the aberrant commissural axons maintained a potential to regrow across the hemispheres at prenatal and early postnatal stages if they were presented with a properly aligned, glia-covered scaffold spanning the hemispheres.