Septal nuclei in primate phylogeny. A quantitative investigation

Specific septal nuclei were quantitatively studied to determine their developmental changes during evolution. The animals utilized were three insectivores (Erinaceus europaeus, Crocidura russula and Sorex araneus), one prosimian (Galago demidovii) and two higher primates (Colobus badius and Cercopithecus ascanius). The primitive insectivores were used as basal forms of reference upon which developmental changes were determined for the progressively evolving primates. Septal nuclei were histologically identified and delineated on histologic photographs made from the serial histologic sections through the septum. The components of the septum were quantitatively compared to both the size of the total septum and to animal body size. The latter quantitative technique known as the allometric method provides measurements for change in absolute size. Increased growth rate occurred in all septal components except for nucleus septalis triangularis and the bed nucleus of the anterior commissure. The structures which underwent the greatest growth rate were the fimbria‐fornix complex, the bed nucleus of the stria terminalis and the nucleus septalis lateralis. The greatest single contributor to the enlargement of the septum in phylogeny was the nucleus of the diagonal band of Broca. Its growth rate was similar to that of the total septum (1.7 × basal forms) and it made up 30% of the septum volume. The nucleus septalis triangularis was the only one of the septal components to undergo an absolute reduction in size. The subfornical body, a structure lying along the ventro‐caudal surface of the ventral hippocampal commissure, underwent a marked increased size in higher forms, 4 × to 6 × that of basal forms.