The cerebrum is the largest part of the brain. Depending upon the position of the animal it lies either in front or on top of the brainstem. In humans, the cerebrum is the largest and best-developed of the five major divisions of the brain. The cerebrum is the newest structure in the phylogenetic sense, and in mammals it is the largest and most developed, out of all known species. The cerebrum is made up of the two cerebral hemispheres and their cortices, (the outer layers of gray matter), and the underlying regions of white matter.