Schematic representation of the spinal commissural and corpus callosum formation. (a) Callosal axons arise from neurons of the cerebral cortex, which turn medially and grow across the dorsal midline of the telencephalon. (b) Dorsally located interneurons of the left and right hemispinal cord extend axons which navigate towards the ventral floor plate in which they cross the midline. After crossing, commissural axons turn rostrally and navigate parallel to the floor plate to reach their target cells.