Evidence that the early postnatal restriction of the cells of origin of the callosal projection is due to the elimination of axonal collaterals rather than to the death of neurons.

By using two fluorescent dyes that are retrogradely transported along axons, we have been able to demonstrate that many of the neurons in the parietal region of the rat cerebral cortex that can be labeled from the contralateral hemisphere early in postnatal development, persist well beyond the period when the callosal projection normally becomes restricted. This indicates that the major factor in the progressive restriction of the callosal projection is the withdrawal or degeneration of axon collaterals, rather than the selective death of many of the cells that initially project to the opposite side.