Transection of ipsilateral ION also arrests target selection of layer II/III callosal neurons. (A) Layer II/III callosal axons project extensively into the contralateral S1/S2 border cortex. (B, C) Transection of ION ipsilateral to callosal neurons at P2 (B) and P5 (C) arrests almost all callosal axon outgrowth into the S1/S2 border region. (D) Ipsilateral transection ION at P7, however, does not obviously affect callosal projections to the border regions. (E) Cortical invasion index (see Methods); *P < 0.05, **P < 0.01. Large triangles point to the S1/S2 border, small triangles point to callosal axons in the white matter beneath the S1/S2 border, and arrows point to boundary regions between two barrels. For abbreviation, see Figure 1. Scale bar, 250 μm.