Afferent connections to the diencephalon in the marsupial phalanger and the question of sensory convergence in the “Posterior Group” of the thalamus

The various afferent pathways terminating in the thalamus of the marsupial phalanger have been investigated with the Nauta techniques, with a view to determining the extent of overlap and convergence of these pathways. If this had proved to be extensive, it would have lent support to the views of some authors that, in primitive and generalized mammals, the relay nuclei are in the process of differentiating out of a single archetypical, multi‐modal nucleus. However, the amount of overlap between the various afferent pathways in this animal is no greater than in the cat and monkey; in the case of the ventral nuclear complex, despite the presence of a common sensory and motor cortical representation, the overlap between the somatic sensory and cerebellar pathways is probaly less than in the higher mammals. The question, therefore, arises as to whether differentiation of certain thalamic nuclei may precede that of the cortical area with which they are connected. The results furnish evidence for the homologies of the posterior group of thalamic nuclei in other mammals, but the limited amount of overlap of afferent pathways occurring in the posterior group is not necessarily indicative of its being the common precursor of all the sensory relay nuclei.