(A-L) Semi-serial transverse cryosections obtained from E12.5 wild-type (left column) or Math1/L1 embryos (right column) that were labeled with a variety of commissural axon markers. In Math1/L1 embryos, L1 is mis-expressed on dorsally located cell bodies of Math1 progenitors and the commissural neurons derived from these progenitors (white arrowheads), as well as on the corresponding axons (arrows) that extend ventrally toward and across the floor plate (B). The distribution of other commissural axon markers is unaffected in Math1/L1 embryos (D, F, H, J, L). Scale bar in (L), 100 μm (A-L).