The mesencephalon is a conspicuous structure in the fishes from its greatly developed roof (_tectum opticum_) which receives the end pencils of the optic nerve. Normally it projects upwards as a pair of large optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is not sufficiently great to cause obvious lateral swellings in external view. [Illustration: FIG. 30.--Median Longitudinal Section through the brain of _Lepidosiren_ and _Polypterus_. In the upper figure (_Lepidosiren_) the habenular ganglion and hemisphere are shown in outline though not actually present in a median section. a.c, Anterior commissure. par, Paraphysis. cer, Cerebellum. pin, Pineal body. d.s, Dorsal sac. p.c, Posterior commissure. g.h, Habenular ganglion. s.v, Saccus vasculosus. h.c, Habenular commissure. t.o, Tectum opticum. i.g, Infundibular gland. v.III, Third ventricle. l.p, Lateral plexus. v.IV, Fourth ventricle. o.c, Optic chiasma. vel, Velum transversum.] pall, Pallium. The thalamencephalon is one of the most interesting parts of the brain from its remarkable uniformity throughout the Vertebrata. Even in _Amphioxus_ the appearance of a sagittal section strongly suggests vestiges of a once present thalamencephalon.[39] The roof--like that of the myelencephalon--remains to a great extent membranous, forming with the closely applied _pia mater_ a vascular roof to the III. ventricle. Frequently a transverse fold of the roof dips down into the III. ventricle forming the _velum transversum_ (fig. 30). The side walls of the thalamencephalon are greatly thickened forming the _thalamus_ (epithalamus and hypothalamus), while a ganglionic thickening of the roof posteriorly on each side forms the _ganglia habenulae_ which receive olfactory fibres from the base of the hemisphere. The habenular ganglia are unusually large in the lampreys and are here strongly asymmetrical, the right being the larger. The floor of the thalamencephalon projects downwards and backwards as the infundibulum. The side walls of this are thickened to form characteristic _lobi inferiores_, while the blind end develops glandular outgrowths (infundibular gland, fig. 30) overlaid by a rich development of blood sinuses and forming with them the _saccus vasculosus_. The optic chiasma, where present, is involved in the floor of the thalamencephalon and forms a large, upwardly-projecting ridge. Farther forwards on the floor or anterior wall is the anterior commissure (see below).