forebrain forebrain in the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. the forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary brain vesicles during the early development of the nervous system. the forebrain controls body temperature, reproductive functions, eating, sleeping, and the display of emotions. forebrain (prosencephalon)diagram depicting the main subdivisions of the embryonic vertebrate brain. these regions will later differentiate into forebrain, midbrain and hindbrain structures.identifiersmeshd016548neuronames27neurolex idbirnlex_1509ta98a14.1.03.006ta25416tee5.14.1.0.2.0.10 fma61992anatomical terms of neuroanatomy at the five-vesicle stage, the forebrain separates into the diencephalon (thalamus, hypothalamus, subthalamus, and epithalamus) and the telencephalon which develops into the cerebrum. the cerebrum consists of the cerebral cortex, underlying white matter, and the basal ganglia. in humans, by 5 weeks in utero it is visible as a single portion toward the front of the fetus. at 8 weeks in utero, the forebrain splits into the left and right cerebral hemispheres. when the embryonic forebrain fails to divide the brain into two lobes, it results in a condition known as holoprosencephaly.