With regard to the relative size and positions of specific types of cortex on the convexity of the brain in whales little is presently known. Sokolov, Ladygina and Supin ( 1972) and Ladygina, Mass and Supin ( 1978) have carried out a series of electrophysiological mapping studies on convexity cortex of the dolphin and a map summarizing their findings is shown in Figs. 1.1E, and 1.7. They have even claimed, on the basis of evoked potential work, to have been able to define both primary and secondary auditory and visual cortex in the dolphin (see Fig. 1.7). We have followed up this work with a cytoarchitectonic analysis of these same areas and, to date, have not on these grounds been able to demonstrate any koniocortices indicative of primary sensory cortex. In Figs. 1.8 and 1.9 we indicate the cytoarchitecture of similar cortical fields mapped by the Russians (as shown in Fig. 1.7). We are presently carrying out Golgi analyses of these same formations and, to date, these have revealed in these areas extremely extraverted neurons of accentuated layer II with dendrites extending at wide angles into accentuated layer 1 (see Fig. 1.5). Our Golgi analyses indicate that layer II is composed largely of immature-type pyramidal neurons with strong development of apical branches into layer I and poorly developed basilar dendrites. In general, our evidence to date in no way distinguishes a primary sensory cortical area on the convexity of the hemisphere. It appears that the parinsular/paralimbic type cortices we have identified as forming the main mass of convexity cortex may contain the equivalents of secondary sensory-motor areas, though none of these are clearly definable by cytoarchitectonic or Golgi analyses.
The whale cortex does not appear to have developed the last or phylogenetically latest stage of cortical evolution characteristic of primates and many other mammals. It is possible that hypergranular cores (koniocortex) and area gigantopyramidalis developed about 50 million years ago in land mammals, whereas whales returned to water some 70-90 million years ago before granularization of the cortex occurred. The leading stage in cortical differentiation in whales seem to be the paralimbic-parinsular stage which is the most primitive of somatomotor representation. It is obvious that Golgi studies combined with evolutionary and ontogenetic analyses, along with physiological approaches, are needed to shed further light on fundamental cortical types and their extent in the whale brain. Though the whale brain has taken a different course of evolution it retains all conservative characters seen in primitive terrestrial and aerial forms such as hedgehogs and bats. Given that the whale returned to water many million of