Homology in the evolution of the cerebral hemispheres. The case of reptilian dorsal ventricular ridge and its possible correspondence with mammalian neocortex.

The present paper reviews some issues related to the evolutionary origin of distinct components of the cerebral hemispheres in vertebrates, which entails the problem of biological homology between anatomical structures. Considering that the term homology is essentially a comparative concept, making emphasis on structural correspondences between organs or body parts, I use the term evolutionary, or phylogenetic homology to denote a common evolutionary origin of two characters. In particular, the controversy of a possible phylogenetic homology between reptilian dorsal ventricular ridge (DVR) and parts of mammalian neocortex is analyzed in some detail. Although it is likely that DVR is a derived character of reptiles while neocortex is a derived character of mammals, the two structures might still originate from the same primordial anlage in the common ancestor. One main problem in the comparison of telencephalic components between reptiles and mammals is that the protrusion of reptilian DVR into the lateral ventricle causes a distortion of the topographic relations in the hemisphere. In order to determine possible homologues of DVR, it is necessary to establish clear-cut telencephalic landmarks. Since lateral cortex is similarly localized in reptiles and mammals, it is suggested that the embryonic position and timing of development of reptilian DVR in relation to lateral cortex may give special insight on the phylogenetic origins of the former. If, as implied by the work of early authors, DVR arose in evolution through an extension of the embryonic period of neuronal proliferation and migration, it may be considered as a genuine novelty in brain evolution. It is also proposed that, regardless of whether DVR and extrastriate neocortex can or cannot be considered phylogenetic homologues, some of the integrative functions performed by them might have a common evolutionary origin, that became localized in reptilian DVR and in mammalian extrastriate neocortex.