Simultaneous bilateral ION transection has no effect but non-simultaneous bilateral transection blocks callosal target selection. (A) Layer II/III callosal axons of control pups project densely to the contralateral S1/S2 border cortex, although axonal arborization is not evident under this artificial nutrition condition (compare with Figures 1A, 3A). (B) This pattern is maintained after bilateral ION transection at P5. (C) Transection of the contralateral ION at P5 and the ipsilateral ION at P7 abolishes callosal innervation of the S1/S2 border region. (D) A similar effect is observed after transecting the ipsilateral ION at P5 and contralateral ION at P7. (E) Cortical invasion index (see Methods); **P < 0.01. Large triangles point to the S1/S2 border, and small triangles point to callosal axons in the white matter beneath the S1/S2 border. For abbreviation, see Figure 1. Scale bar, 250 μm.