The term heterotypic cortex refers to those areas of mature cerebral cortex that deviate markedly from the homogeneous six- layered internal structure seen in the third trimester of gestation. heterotypic cortex is contrasted to homotypic cortex, which retains the fetal six-layered pattern into adulthood. Together they constitute the neocortex. A limited number of neocortical areas undergo modification to more than six layers. They include the primary visual area ( primary visual cortex ), in which both the internal granular layer (IV) and the external pyramidal layer (III) divide into two or three sublayers, and the insula, if one considers the underlying claustrum to have originated during development from the multiform layer (VI), enlarged, and become separated from the insula by fibers of the extreme capsule. Other heterotypic areas are characterized by a decrease in layers, for the most part due to the merger or absence of the external granular layer and/or the internal granular layer. They include the motor cortex ( area 4 of Brodmann (human), area 6 of Brodmann (human), area 8 of Brodmann (human) ); the anterior cingulate gyrus ( area 24 of Brodmann (human), area 25, area 33 of Brodmann); the retrosplenial area ( area 30 of Brodmann (human) ) and the perirhinal cortex ( area 35 of Brodmann ); the agranular insula, and their equivalents in other species. The the specific areas of heterotypic cortex differ somewhat by species ( Brodmann-1909 ).