Interspecies comparison of the hippocampal formation shows increased emphasis on the regio superior in the Ammon's horn of the human brain.

The cells in the hippocampal formation of rat, cat, rhesus monkey and human have been counted. The results show that there is a continuous increase both in granule and pyramidal cell numbers from rat to human. However, in human there is a disproportionate increase in the number of pyramidal cells of the regio superior. In the dentate gyrus the hilar region accumulates higher number of cells than it might have been expected from the average increase.