In mammals comparable lesions of the cerebral cortex affecting motor status have far less permanent and severe effect when the injury is sustained in infancy than when it occurs in maturity.1This is apparent in all species in which adult motor performance is largely dominated by the cerebral cortex, as in man, monkey, dog and cat.2Thus, when the cortical areas 4 and 6 of Brodmann are removed from infant monkeys or chimpanzees, there is little immediate effect on motor performance and only moderate effects appear as such animals later develop.3In contrast, adult animals exhibit severe paresis after similar procedures (figs. 1 and 2). The processes, anatomic or physiologic, which lie behind this striking age difference are unknown. Even the relationship of myelination to function is not certain, although much discussed. This paper deals with one obvious and immediate step in further analysis of the question,