Quantitative composition and histological features of the brain in two South American edentates.

The volumes of twelve brain components relative to body weights in the armadillo and the tree-sloth have been estimated. The progression of each component with respect to a basal reference type is indicated. The armadillo shows the brain organization of a little evolved mammal with developed olfactory structures and a large pyriform lobe in particular. The sloth possesses a more advanced type of brain with an unexpectedly voluminous neocortex. Some behavioural features probably correlated to the size of brain structures in those Edentates are mentioned and the need for a reconsideration of the concept of primitiveness is stressed.