50 THE ORIGIN OF VERTEBRATES 
In the whole evolution from the brain of Ammoccetes 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 con- 
striction 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 fimbrie, is already 
commencing to grow from the ventral mass of 
the medulla oblongata to surround these lateral 
membranous folds. These fimbriee develop more 
seu et, eaubat and more in higher forms, and thus form the 
lum; IV., membra- cerebellar hemispheres. 
nous roof of fourth Not only does comparative anatomy confirm 
ies a gana es the teachings of embryology, but also pathology 
nous folds on each gives its quota in the same direction. 
side. Through these One of the striking facts about malforma- 
fee wis By) “" tions 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 
Fie, 24, — CEREBEL- 
LUM OF DoG-FISH. 
