We confirmed the formation of an aberrant ipsilateral corticospinal tr
act after unilateral cerebral cortical ablation during the neonatal pe
riod in rat. This tract was studied using anterograde horseradish pero
xidase (HRP) tracing. Ramifications of the axons in the pyramidal trac
t were found to contribute to the ipsilateral tract at the level of th
e pyramidal decussation, suggesting that ramification of immature axon
s plays an important role in the formation of the ipsilateral corticos
pinal tract.