The cerebrum or telencephalon, together with the diencephalon, constitutes the fore-brain. It encompasses about two-thirds of the brain mass and lies over and around most of the structures of the brain. The outer portion of the cerebrum is covered by a thin layer of gray tissue called the cerebral cortex. In humans, the cerebrum is the most superior region of the vertebrate central nervous system. However, in the majority of animals, the cerebrum is the most anterior region of the CNS as the anatomical position of animals is rarely in the upright position. Telencephalon refers to the embryonic structure from which the mature cerebrum develops. In mammals, the dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into approximately symmetric left and right cerebral hemispheres.