The Cerebral Cortex of Reptiles

Reptiles and mammals are the two groups of vertebrates with well-developed cerebral cortices. Ray-finned fishes have forebrains that develop by an eversion of the rostral neural tube that reduces the roof of the telencephalon to a thin membrane (Nieuwenhuys, 1982; Northcutt and Davis, 1983). They have olfactory cortices on the ventrolateral walls of their cerebral hemispheres (e.g., Braford and Northcutt, 1974), but the eversion process seemingly precludes the formation of a cortical roof to the telencephalon. Cartilaginous fishes (such as sharks), fleshy-finned fishes (lungfishes and coelacanths), amphibians, reptiles, birds, and mammals all have forebrains that develop via a fundamentally different process. This involves an evagination of the rostral neural tube resulting in paired lateral ventricles, interventricular foramina, and a neuronal roof to the telencephalon that can form an extensive cerebral cortex. However, the telencephalic roof of sharks forms a solid mass of neurons lacking the lamination usually associated with the cerebral cortex (Smeets et al., 1983). Although amphibians (Northcutt and Kicliter, 1980) and lungfishes (Northcutt, 1986) have laminated cortices, they show little migration of neurons away from the ependyma. Birds have a small cerebral cortex, ostensibly resulting from a secondary reduction of the reptilian pattern (Benowitz, 1980). It is only in reptiles and mammals that the telencephalic roof develops into extensive and multilayered cortices.