THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 49 
region of the brain up to the region of the pineal eye and ganglion 
habenule is one large membranous bag, except for the single con- 
striction where the fourth nerve on each side crosses over. The 
explanation of this peculiarity is given in Chapter VII., and follows 
simply from the facts of the arrangement of that musculature in the 
scorpion-group which gave rise to the eye-muscles of the vertebrate. 
In Ammoccetes both cerebellum and posterior corpora quad- 
rigemina 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 Ammoccetes 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 commis- 
sures in every vertebrate. This naturally is simply the commissure 
between the two supra-cesophageal 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 Ammoccetes, 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. 
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