Corpus callosum connections of parietal and motor cortex were studied in New World owl monkeys (Aotus trivirgatus) and Old World macaque monkeys (Macaca fascicularis) after multiple injections of 3H-proline and horseradish peroxidase, HRP, into one cerebral hemisphere, and extensive microelectrode mapping of architectonic Areas 3b, 1, and 2 of the other hemisphere. Results were obtained both from parasagittal brain sections cut orthogonal to the brain surface and from sections from flattened brains cut parallel to the brain surface. Cortical fields varied in density of callosal connections, and the density of connections varied according to body part within sensory representations. Thus, Area 3b had few, Area 1 had more, and Area 2 had relatively dense callosal connections. Within each of these fields, connections were much less dense for the representations of the glabrous hand and foot and much more dense for the representations of the face and trunk. For the representation of the hand, retrogradely labeled cells were extremely sparse in Area 3b, moderately sparse in Area 1, and moderate in Area 2. There were less dense callosal connections in the hand representations of Areas 3b, 1, and 2 in macaque as compared to owl monkeys. Label in posterior parietal cortex was uneven with zones of extremely dense connections. A large region of very dense callosal connections was noted in motor cortex just medial to the probable location of the hand representation. In all regions, callosally projecting cells appeared to be more broadly distributed than callosal terminations. In no region was the discontinuous arrangement of callosal connections obviously organized into an extensive pattern of mediolateral or rostrocaudal bands or strips.

Development changes in the origins of contra- and ipsilateral projections from the rat parietal cortex were examined using the retrograde transport of horseradish peroxidase and of the fluorescent dyes fast blue and nuclear yellow. The aim of the study was to determine the fate of the neurons which send axons through the corpus callosum in the neonatal, but not the young adult, rat. 1. Neurons which project across the corpus callosum at postnatal day 8 (PND 8), but not at PND 20 or later, do not die; they merely lose their contralateral processes. 2. At PND 8, many of these neurons--in particular, those in lamina Va of the barrel field area--project simultaneously across the corpus callosum and to the ipsilateral motor cortex. 3. By PND 20 and later, many neurons in lamina Va of the barrel field area still project to the ipsilateral motor cortex but not across the corpus callosum. We conclude that at least some neocortical neurons initially elaborate more axonal processes than they will maintain and that they selectively eliminate major projections during development.