It continues to flex at the mesencephalon. Grades 10 to 12 were aprosopus with absence of the prosencephalon and mesencephalon. The TS is located in the mesencephalon. Initially there are three primary brain vesicles&colon; prosencephalon, mesencephalon, and rhombencephalon. It is located in the mesencephalon below the interpeduncular fossa. Wohlfart "et al" hypothesized that klazomania originates in the periaqueductal gray matter in the mesencephalon. In the human brain, the telencephalon covers most of the diencephalon and the mesencephalon. The sulcus limitans extends the length of the spinal cord and through the mesencephalon. The diencephalon, mesencephalon and rhombencephalon constitute the brain stem of the embryo. These swellings represent different components of the central nervous system&colon; the prosencephalon, mesencephalon and rhombencephalon. The mesencephalon is considered part of the brainstem. Premotor cells for fusional vergence are located in the mesencephalon near the oculomotor nucleus. The midbrain or mesencephalon contains the two optic lobes. The oculomotor nerve (CN III) arises from the anterior aspect of mesencephalon (midbrain). It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.