Growth and reshaping of axons in the establishment of visual callosal connections

The visual cortical areas in the two hemispheres are interconnected by axons running through the corpus callosum. In adult cats, these axons originate from, and terminate in, tangentially restricted portions of each area. In young kittens, however, callosal axons originate from the entire extent of each area, although they apparently enter the gray matter only in the restricted regions where they will also be found in adults. In kittens, but not in adults, callosal axons also reach other regions, but there they appear to be confined to the lowest part of layer VI. During the first two postnatal months, the callosal efferent zones become progressively restricted to their adult locations. During this process, many neurons eliminate the axons (or axon collaterals) that they had formerly sent through the corpus callosum and form permanent connection ipsilaterally.