Some histochemical observations on the telencephalon of the bullfrog, Rana catesbeiana Shaw.

The histochemically determined distribution of acetylcholinesterase, monoamine oxidase and succinate dehydrogenase in the telencephaIon of the bullfrog supports the classically recognized divisions of the pallium and subpallium. Analysis also corroborates the following generally recognized anuran-amniotic homologies: anuran medial pallium to amniotic medial cortex, anuran septal nuclei to amniotic septal nuclei, anuran striatum to amniotic corpus striatum. On topographical and histochemical criteria the ventrocaudal and basomedial portions of the anuran telencephalon are considered possible homologues to the mammalian amygdala. It is suggested that two divisions can be recognized: a pars lateralis which may be homologous to the mammalian cortico-medial group, and a pars medialis which may be homologous to the mammalian baso-lateral group. Further analysis suggests, particularly when viewed in the light of recent experimental anatomical studies, that the anuran lateral pallium consists of a pars dorsalis and a pars ventralis. The pars dorsalis may be the reptilian homologue of the dorsal cortex and the pars ventralis may be the field homologue of both the reptilian piriform cortex and the dorsal ventricular ridge. Recent advances in neurohistochemistry have significantly increased the armamentarium with which neuroanatomists search for clues to central nervous system homologies and have thus provided a better understanding of vertebrate brain evolution. The availability of a new investigative method is particularly useful for application to the complexities of telencephalic organization. The variation of this structure among vertebrates is extensive, and as homologies are pragmatically based on similarities between structures, the recognition of telencephalic homologies has, at best, been extremely difficult. The past 100 years have demonstrated that no single method has yet been the answer to unravelling homologies in this brain region, nor is histochemistry likely by itself to be a neural Rosetta Stone. It is, however, a welcome tool that can yield data greatly facilitating the identification of telencephalic homologies. Perhaps the most striking difference in the telencephalic organization of most anamniotic and amniotic vertebrates is the elaboration in amnioties of a dorsal ventricular ridge. In reptiles and birds the J. COMP. NEUR., 157. 379-390. dorsal ventricular ridge appears to form by a proliferation of the lateral pallial wall (Johnston, '16; Kuhlenbeck, '38; Kallen, '62). This proliferation of cells almost obliterates the lateral ventricle and results in a mass of neural tissue with a variable number of recognizable cytological fields in different taxa (Karten and Hodos, '67; Northcutt, '67, '70a). Despite this cytological variability, all amniotic taxa that have been examined experimentally possess a dorsal ventricular ridge which receives discrete thalamic projections related to vision and audition (Karten, '68, '70; Hall and Ebner, '70; Pritz, '72, '73). Comparisons between sauropsid vertebrates and mammals based on a wide variety of techniques suggest that the sauropsid dorsal ventricular ridge is homologous to part of mammalian isocortex (Kalen, '62; Karten, '69; Northcutt, '69a,b; Nauta and Karten, '70; Parent and Olivier, '70). Does the telencephalon of living amphibians possess a field of neurons homologous to the sauropsid dorsal ventricular ridge? If so, what is its topographical position and what are its connections with other forebrain structures? In an attempt