A second auditory area in the non-cortical telencephalon of a reptile.

Injections of horseradish peroxidase (HRP) into a caudocentral portion of the non-cortical telencephalon of Caiman known as the dorsolateral area (dorsal ventricular ridge) resulted in retrogradely labeled neurons throughout the entire extent of the ipsilateral nucleus reuniens. HRP-positive cells were most numerous in nucleus reuniens pars diffusa with only sparse labeling of neurons in nucleus reuniens pars centralis. The results of the present experiment, when compared with those of a prior study that determined the telencephalic connections of nucleus reuniens pars centralis, suggested that these two forebrain areas are separate. Staining with succinate dehydrogenase and acetylcholinesterase revealed that nucleus reuniens pars centralis and pars diffusa and their respective telencephalic projection areas can be differentiated on the basis of histochemical features. These findings in Caiman suggest that certain thalamic and telencephalic auditory areas in birds and crocodilians are most likely the result of common ancestry rather than examples of parallel evolution.