In Ammocœtes both cerebellum and posterior corpora quadrigemina can hardly be said to exist, but upon transformation a growth of nervous material takes place in this region, and it is seen that this commencing cerebellum and the corpora quadrigemina arise from tissue that is present in Ammocœtes along the course of the fourth nerve.
Here, then, again Embryology does its best to tell us how the vertebrate arose. The formation of the two primary constrictions in the dilated anterior vesicle whereby the brain is divided into fore-brain, mid-brain, and hind-brain is simply the representation ontogenetically of the two nerve-tracts which crossed over the cephalic stomach in the prevertebrate stage, in consequence of the mid-dorsal position of the pineal eyes and of the insertion of the original superior oblique muscles.
The subsequent constriction by which the prosencephalon is separated from the thalamencephalon is in the position of the anterior commissure, that commissure which connects the two supra-infundibular nerve-masses, and is one of the first-formed commissures in every vertebrate. This naturally is simply the commissure between the two supra-œsophageal ganglia; anterior to it, in the middle line, equally naturally, the anterior end of the old stomach wall still exists as the lamina terminalis.
The other division in the hind-brain region, which separates the region of the cerebellum from the medulla oblongata, is due to the growth of the cerebellum, and indicates its posterior limit. In such an animal as the lamprey, where the cerebellum is only commencing, this constriction does not occur in the embryo.
From such simple beginnings as are seen in Ammocœtes, 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.
In the whole evolution from the brain of Ammocœtes 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.
Fig. 24.—Cerebellum of Dog-fish.