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Quantitative studies of the evolution of the thalamus in primates.

A quantitative study of the thalamus and metathalamus has been carried out on brains of six different primate species. The allometric formula devised by Stephan, Bauchot and Andy (1970) has been slightly modified as two steps have been utilized in order to attain the required magnification of the final photomicrograph (a positive print, as opposed to Stephan et al's negative print). The thalamic and metathalamic structures are divided on the basis of their developmental stages into two groups--paleothalamus and neothalamus, so that their proportions to each other in primate phylogeny can be estimated. The results show that the neothalamus extends the paleothalamus by two-thirds (2/3rds) in Tupaia and by more than three-quarters (3/4ths) in Cercopithecus. Of all the thalamic groups, the dorsolateral thalamic nuclear group is most dominant in all primate species, except Tupaia, where the ventrolateral thalamic nuclei are larger than all other nuclei. The pulvinar is the most dominant nucleus in all species except Tupaia. These quantitative results conform, in most places, with the observations of other researchers, while they do not agree much with the qualitative observations of the thalamus in the same species (Simmons 1974). How this study has a bearing on the phylogenetic positions of the Tupaioidea and the Tarsioidea in the Primate Order, is briefly discussed.