Quantitative changes in the fundamental structural pattern of the diencephalon among primates and insectivores.

The volumes of the main components of the diencephalon from 10 basal insectivores, 12 prosimians, and 15 simians including man were compared. The comparison of the percentage composition within the diencephalon shows that the thalamus and the subthalamus increase whereas the epithalamus and the hypothalamus decrease in relative size among primates. The allometric analysis reveals that, in spite of a relative reduction, the epithalamus and the hypothalamus are also progressive structures. However, their allometric size increase, being especially accentuated in earlier phyletic phases, is small in comparison to that of the thalamus and the subthalamus. These latter two components are characterized by a continuous phylogenetic growth from the basal insectivores to the simians. The correlation analysis shows that the strongest correlations and, in many cases, that isometric size increase exist between structures which belong to the same functional system. These findings indicate the existence of at least two major developmental gradients in the quantitative development of the diencephalon, a neencephalic and a limbic one.