The areas and layers of corticocortical terminations in the visual cortex of the Virginia opossum

Visual cortex in the opossum was defined operationally as striate cortex and the total cortical area receiving ipsilateral projections from striate cortex. This area included all of Gray's ('24) peristriate cortex. In this study several areas were identified within Gray's striate and peristriate areas on the basis of changes in cytoarchitecture and parallel changes in thalamocortical fiber termination patterns. The layers of termination within these cortical areas, of the ipsilateral associational fibers, the commissural fibers and the thalamocortical fibers from the hemithalamus were determined by means of the Fink‐Heimer stain. (1) Commissural fiber terminations occur in individual dense “bands” in certain regions of the different cytoarchitectural areas of visual cortex. These bands of commissural fiber terminations are separated by zones of cortex which receive only sparse commissural fiber terminations. The first band of commissural terminations is found in a zone of transitional cortex occupying lateral striate cortex and adjacent peristriate cortex. This band is characterized by terminations in all layers of cortex on the peristriate side and terminations circumscribing the lateral edge of layer IV on the striate side. A second commissural band of terminations lies in anterior and central peristriate areas with terminations in all layers except V. A cytoarchitectural delineation is present in central peristriate cortex between these two bands of commissural terminations. A third band with a laminar pattern of commissural terminations similar to the second band lies more laterally in other peristriate areas along the rhinal fissure and temporal cortex. These three bands broaden at their midpoints to fuse with one another. Commissural terminations are also found in a strip of peristriate cortex on the medial surface of the hemisphere. Characteristic of all commissural terminations in layer I is their restriction to the inner three‐fourths of this layer. Subtotal lesions of visual cortex reveal homotopic and heterotopic commissural connections. Central striate cortex has few commissural interconnections. Most of this area receives a few commissural fiber terminations from the lateral portion of striate cortex and also from peristriate cortex. Fibers from the lateral region of striate cortex give rise to terminations which form the band about the striate border as may peristriate cortex adjacent to the lateral striate cortex border. Medial peristriate cortex has homotopic interconnections while the other peristriate areas have both heterotopic and homotopic interconnections. (2) the ipsilateral associational projections from striate cortex terminate mainly in layers I to IV in peristriate cortex. These terminations overlap the zones of peristriate cortex which receive sparse and dense commissural terminations. Degeneration in the lesioned hemisphere revealed tangential fibers in layer I that are present over a large extent of visual cortex. Thermal lesions of only layers I and II of striate cortex show that at least part of the ipsilateral association termination pattern arises from layer II. (3) In each cytoarchitectural area the corticocortical terminations are compared with the thalamocortical terminations and also with such axonal distributions described by Golgi and EM studies in other mammals.