Gyrus formation in the cerebral cortex in the ferret. I. Description of the external changes.

The external features of gyrus formation in the postnatal ferret cerebral cortex are described and correlated with certain internal changes. The observations indicate that gyri are formed by longitudinal and radial expansion of the cortical compartment occurring between relatively fixed areas which form the sulcal floors. The gyri were initially rounded with open sulci and the cerebrum had a rectangular outline when seen in lateral and dorsal view. By adult life the hemisphere had been subjected to considerable moulding by the growing skull, so that the frontal pole of the cerebrum became pointed while the sulcal walls became closely opposed and the gyral crowns flattened.