The available data concordantly suggest that the mammalian thalamus consists of three divisions different from each other in their phylogenetic and ontogenetic development and in their relations to the cortex. The epithalamus (paraventricular complex, habenular complex, and the pretectal group of nuclei) is entirely independent of the endbrain in all mammals and undergoes a strong reduction in higher forms. The dorsal thalamus is entirely dependent on the endbrain. Each nucleus of this division has a restricted projection upon the endbrain without which it cannot survive. The dorsal thalamic nuclei are classified as extrinsic or intrinsic depending on whether or not they receive a substantial portion of their afferents from extra-thalamic sources. It can be shown that the neocortex of primitive mammals consists largely of projection areas of extrinsic thalamic nuclei (primary cortical areas) whereas in the neocortex of higher forms the projection areas (secondary cortical areas) of the intrinsic thalamic nuclei become dominant. The intrinsic thalamic nuclei are separable into two groups. Those projecting upon the neocortex become dominant in primates, whereas the intrinsic nuclei projecting upon the rhinencephalic structures are on the whole best developed in macrosmatic mammals. The ventral thalamus consists of one subdivision (ventral lateral geniculate body) entirely independent of the endbrain and of a second subdivision (reticular complex) which projects upon a large number of cortical fields. The sparse and generalized - though spatially well organized - projection of the reticular complex provides a system apparently independent of the dorsal thalamic projections and capable presumably, of evoking generalized cortical activity.