A) Transverse section of the mouse spinal cord at embryonic day 11.5. Pre-crossing spinal commissural neurons navigate ventromedially and express the cell adhesion molecule Tag1. B) Transverse section of the mouse spinal cord at embryonic day 11.5. Post-crossing commissural neurons express the cell adhesion marker L1. C) Open book preparation of the mouse spinal cord at embryonic day 11.5. Spinal commissural neurons are labeled by DiI injection into the dorsal spinal cord. The majority of post-crossing commissural axons turn anteriorly. The bracket indicates the position of the floor plate. D) Three segments of the Drosophila stage 16 embryonic ventral nerve cord. MAb BP102 (magenta) labels all axons in the central nervous system. egGal4 drives GFP (green) expression in a subset of commissural neurons. FP, floor plate. LF, lateral funiculus. VF, ventral funiculus. AC, anterior commissure. PC, posterior commissure. Panels A-C are reprinted from reference 67, with permission from Elsevier.