<P> The cerebrum is divided by the medial longitudinal fissure into two cerebral hemispheres, the right and the left . The cerebrum is contralaterally organized, i.e., right hemisphere controls and processes signals from the left side of the body, while the left hemisphere controls and processes signals from the right side of the body . There is a strong but not complete bilateral symmetry between the hemispheres . The lateralization of brain function looks at the known and possible differences between the two . </P> <P> In the developing vertebrate embryo, the neural tube is subdivided into four unseparated sections which then develop further into distinct regions of the central nervous system; these are the prosencephalon (forebrain), the mesencephalon (midbrain) the rhombencephalon (hindbrain) and the spinal cord . The prosencephalon develops further into the telencephalon and the diencephalon . The dorsal telencephalon gives rise to the pallium (cerebral cortex in mammals and reptiles) and the ventral telencephalon generates the basal ganglia . The diencephalon develops into the thalamus and hypothalamus, including the optic vesicles (future retina). The dorsal telencephalon then forms two lateral telencephalic vesicles, separated by the midline, which develop into the left and right cerebral hemispheres . Birds and fish have a dorsal telencephalon, like all vertebrates, but it is generally unlayered and therefore not considered a cerebral cortex . Only a layered cytoarchitecture can be considered a cortex . </P> <P> Note: As the cerebrum is a gross division with many subdivisions and sub-regions, it is important to state that this section lists the functions that the cerebrum as a whole serves . See main articles on cerebral cortex and basal ganglia for more information . The cerebrum is a major part of the brain, controlling emotions, hearing, vision, personality and much more . It controls all voluntary actions . </P> <P> The cerebrum directs the conscious or volitional motor functions of the body . These functions originate within the primary motor cortex and other frontal lobe motor areas where actions are planned . Upper motor neurons in the primary motor cortex send their axons to the brainstem and spinal cord to synapse on the lower motor neurons, which innervate the muscles . Damage to motor areas of cortex can lead to certain types of motor neuron disease . This kind of damage results in loss of muscular power and precision rather than total paralysis . </P>

What structure is described in your text as the gray matter that forms the top portion of the brain