As the spinal cord passes forward into the skull it enlarges and merges with the posterior portion of the brain, the _medulla oblongata_ (_myelencephalon_). The roof of the medulla is extremely thin, and is broken if the cartilage has not been removed with extreme care, exposing a cavity within, the _fourth ventricle_. In front of the medulla, and overlapping its anterior extremity, is a large oval organ, the _cerebellum_ (_metencephalon_). Ventral to the cerebellum, each side of the medulla is expanded in an ear-shaped lobe, the _corpus restiformis_. Anteriorly, the cerebellum overlaps a pair of rounded lobes, the _optic lobes_, which together form the dorsal portion of the _midbrain_ (_mesencephalon_). In front of the optic lobes are two slightly larger lobes united in their posterior portions but separated anteriorly, the _cerebral lobes_ or _hemispheres_. Together they constitute the _prosencephalon_. (The prosencephalon is not divided in Eugaleus.) Between the mesencephalon and the prosencephalon is a depressed region belonging to the brain-stem, the _diencephalon_ (_thalamencephalon_), from which the epiphysis arises. The roof of the diencephalon also is very thin and is frequently broken during the exposure of the brain. The cavity seen within the diencephalon is the _third ventricle_. Stalked bodies arising from the antero-lateral angles of the cerebral hemispheres are the _olfactory lobes_. The portion of the brain including the cerebral hemispheres and the olfactory lobes constitutes the _telencephalon_.