The term area 21 of Brodmann (guenon) refers to a subdivision of the cerebral cortex of the guenon defined on the basis of cytoarchitecture. It is cytoarchitecturally homologous to area 21 of Brodmann (human) ( Brodmann-1909 ). Distinctive features ( Brodmann-1905 ): Compared to area 20 of Brodmann (guenon), the total cortical thickness of area 21 is greater, the granular cells are less abundant, and the boundary with the subcortical white matter is less distinct; the molecular layer (I) is wider; the pyramidal cells of sublayer 3b of the external pyramidal layer (III) are larger; the internal granular layer (IV) is less developed and contains fewer cells; ganglion cells of the internal pyramidal layer (V) are larger, rounder, and densely arrayed adjacent to layer IV; the boundary between layer V and the multiform layer (VI) is indistinct; and layer VI is wider and has no sublayers.