The number of neuronal cell bodies has been counted in a narrow strip (30 micrometers) through the depth of the neocortex in several different functional areas (motor, somatic sensory, area 17, frontal, parietal and temporal and in many species (mouse, rat, cat, monkey and man). With the exception of area 17 of the visual cortex in a number of primates the same absolute number (congruent to 110) of neurons has been found in all areas and in all species. In the binocular part of area 17 of the primates there are approximately 2.5 times more neurons. Thus in mammalian evolution the area of the neocortex increases in larger brains but the number of neurons through the depth remains constant, except in area 17 of primates. From these and other findings it is suggested that the intrinsic structure of the neocortex is basically more uniform than has been thought and that differences in cytoarchitecture and function reflect differences in connections.