Nuclear organization of the bullfrog diencephalon.

A cytoarchitectonic analysis was performed on the diencephalic nuclei of the bullfrog, Rana catesbeiana. The epithalamus contains two widely recognized habenular nuclei. The thalamus has three subdivisions: dorsal and ventral thalamus, and posterior tuberculum. The dorsal thalamus may be further parcelled into anterior, middle, and posterior zones. Connectional data from other studies support this zonation. The anterior zone projects to the telencephalic pallium. The middle zone nuclei receive a strong input from the midbrain roof and project to the telencephalic striatal complex. The posterior zone nuclei do not appear to project to the telencephalon; they may eventually be placed in the pretectum, a transitional area between the diencephalon and mesencephalon. Two of the ventral thalamic populations have been frequently placed in the dorsal thalamus and called the nucleus rotundus and the lateral geniculate nucleus. These terms imply homology with sauropsid dorsal thalamic nuclei, but our analysis and current connectional information do not support such homologies. We have given these populations more neutral names. The hypothalamus is divisible into a preoptic and infundibular hypothalamus, and the preoptic area can be further separated into anterior and posterior preoptic areas. The posterior area contains the magnocellular preoptic nucleus and a dorsal arm of this nucleus, often placed in the ventral thalamus, was recognized. We have tentatively placed the posterior entopeduncular nucleus in the hypothalamus.