The cerebral cortex is the highest achievement of biological evolution and the neural substrate of human mental abilities. Molecular biological techniques have opened the possibility of investigating how specific regulatory genes and morphoregulatory molecules mediate formation of this complex structure. During evolution, the cerebral cortex has undergone disproportionate growth relative to the rest of the brain [reviewed by Northcutt and Kaas (Northcutt and Kaas, 1995)]. This increase predominately affected its surface area, rather than its thickness, and was accompanied by the addition and elaboration of functional subdivisions. Furthermore, the [deletion] region that has increased in size the most, the neocortex, acquired a characteristic laminar structure that is not readily apparent in the cortex of non-mammalian vertebrates. These changes in the size, complexity and histological organization of the cerebral cortex presumably reflect the elaboration of the cortical circuitry which ultimately endowed the human brain with its extraordinary capacity for reasoning and language.