(a) Images with higher magnification of the midline show a series of experiments, wherein Venus-GFP was electroporated into the developing cortex at E12.5 and the brains harvested and analysed at E18.5. Electroporation of Venus-GFP shows a normal CC in wild-type and Ctip2−/− embryos. Satb2−/−-mutant brains clearly demonstrate a complete absence of callosal fibres, wherein no fibres could be seen approaching the midline. Partial rescue of the CC could be seen in Satb2−/−; Ctip2−/− compound mutants. Fibres manage to reach and cross the midline (arrows). The sections have been subjected to immunohistochemical staining for the neural cell adhesion molecule L1 and for GFP. (b) Images show the result of Unc5C/EGFP overexpression in wild-type and Satb2−/−-mutant cortex. Overexpression of Unc5C in Satb2−/− partially restores the CC where axons of electroporated neurons reach and cross the midline (arrow). Normally projecting callosal axons were observed in the control cortex. Scale bar, 450 μm for low-magnification images and 100 μm for high-magnification images.