The different cytoarchitectonic Bautypen of the cerebral neocortex as defined by von Economo (1925, 1927). In the homotypical isocortex (2, 3, 4) the six layers can well be recognized in the Nissl picture. It shows three major types: the frontal (2), the parietal (3), and the polar type as found in the frontal and occipital pole (4). On both sides of these types of the homotypical isocortex extremes are found, in which the six-layered structure is difficult to recognize in the Nissl picture. These heterotypical isocortical areas may either show extremely few small cells (agranular cortex as found in the area praecentralis, 1) or an extreme abundance of small cells (the granular or koniocortex as found in area striata or other sensory cortices 5). Whereas the two heterotypical cortical structures are typical for the efferent (motor, 1) and the afferent (sensory, 5) cortical areas, respectively, the six layered isocortex serves the commemorative and associative functions of the cortex.

From here.