Within the skull, the cerebrum occupies the uppermost region. The two hemispheres of the cerebrum are separated by a deep groove called the longitudinal cerebral fissure, and each of the sections is divided into an inner core of white matter and an outer layer of gray matter known as the cerebral cortex. Located deep inside of the white matter are clustered nuclei termed basal nuclei that appear to be primarily concerned with the planning of sequences of movement and learning to perform them again. The site of most activity in the cerebrum, however, is the cerebral cortex. In mammals, the cerebral cortex is large and complex, while only the most basic components of this region of the brain are present in reptiles.