From such simple beginnings as are seen in Ammocoetes, the higher forms of brain have been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still remain being tucked away into the cavities to form the various choroid plexuses. {50}In the whole evolution from the brain of Ammocoetes to that of man, the same process is plainly visible, viz. growth and extension of nervous material over the epithelial tube; extension dorsally and posteriorly of the supra-infundibular nervous masses (as seen in Fig. 19), combined with a dorsal growth of parts of the infra-infundibular nervous masses to form the cerebellum and posterior corpora quadrigemina. Especially instructive is the formation of the cerebellum. It consists at first of a small mass of nervous tissue accompanying the fourth nerve, then by the growth of that mass surrounding and constricting a fold of the membranous roof, the _worm_ of the cerebellum is formed, as in the dog-fish. This very constriction causes the membrane to be thrown into a lateral fold on each side, as seen in Fig. 24, and in the dog-fish the nervous material on each side, known as the fimbriæ, is already commencing to grow from the ventral mass of the medulla oblongata to surround these lateral membranous folds. These _fimbriæ_ develop more and more in higher forms, and thus form the cerebellar hemispheres. Not only does comparative anatomy confirm the teachings of embryology, but also pathology gives its quota in the same direction. [Illustration: FIG. 24.--CEREBELLUM OF DOG-FISH. _v_, worm of cerebellum; _IV._, membranous roof of fourth ventricle continuous with the membranous folds on each side. Through these the fimbriæ (_fb._) can be dimly seen.] One of the striking facts about malformations and disease of the central nervous system is the frequency of cystic formations; _spina bifida_ is a well-known instance. These cysts are merely epithelial non-nervous cysts formed from the epithelium of the central canal, difficult to understand if the whole nerve tube is one and entirely nervous, either actually or potentially, but natural and easy if we are really dealing with a simple epithelial tube on the outside of which the nervous material was originally grouped. The cystic formation belongs naturally enough to this tube, not to the nervous system. Again, where animals such as lizards have grown a new tail, owing to the breaking off of the original one, it is found that the central canal extends into this new tail for some distance, but not {51}the nervous material surrounding it; all the nerves supplying the new tail arise from the uninjured spinal cord above, the central canal with its lining layer of epithelial cells alone grows into the new-formed appendage.