Further, the brain of Ammocoetes (Fig. 20) shows clearly not only that it is composed of two parts, an epithelial tube and a nervous system, but also that the nerve-masses are arranged in the same relative position with respect to this tube as are the nerve-masses in the invertebrate with respect to the cephalic stomach and oesophagus. This evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the tube did not originate as part of the central nervous system, but was originally independent of the central nervous system, and has been invaded by it. {40}[Illustration: FIG. 19.--COMPARISON OF VERTEBRATE BRAINS. _CB._, cerebellum; _PT._, pituitary body; _PN._, pineal body; _C. STR._, corpus striatum; _G.H.R._, right ganglion habenulæ. _I._, olfactory; _II._, optic nerves.] {41}[Illustration: FIG. 20.--BRAIN OF AMMOCOETES. A, dorsal view; B, lateral view; C, ventral view. _C.E.R._, cerebral hemispheres; _G.H.R._, right ganglion habenulæ; _PN._, right pineal eye; _CH_2_, _CH_3_, choroid plexuses; _I.-XII._ cranial nerves; _C.P._, _Conus post-commissuralis_.] {42}The second explanation is hardly worth serious consideration, for it supposes that the nervous system, for no possible reason, was laid down in its most important parts--the brain-region--as an epithelial tube with latent potential nervous functions; that even up to the highest vertebrate yet evolved these nervous functions are still in abeyance over the whole of the choroid plexuses and the roof of the fourth ventricle. Further, it supposes that this prophetic epithelial tube originally developed into true nervous material only in certain parts, and that these parts, curiously enough, formed a nervous system absolutely comparable to that of the arthropod, while the dormant prophetic epithelial part was formed so as just to mimic, in relation to the nervous part, the alimentary canal of that same arthropod. The mere facts of the case are sufficient to show the glaring absurdity of such an explanation. This is not the way Nature works; it is not consistent with natural selection to suppose that in a low form nervous material can be laid down as non-nervous epithelial material in order to provide in some future ages for the great increase in the nervous system. Every method of investigation points to the same conclusion, whether the method is embryological, anatomical, or pathological.