50 



THE ORIGIN OF VERTEBRATES 



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. 



Espceially 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 fimbriae, is already 

 commencing to grow from the ventral mass of 

 the medulla oblongata to surround these lateral 

 membranous folds. These fimbriae 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. 



One of the striking facts about malforma- 

 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, 

 dilficult 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 



Fig. 24. — Cebebel- 

 lum of Dog-fish. 



v, worm of cerebel- 

 lum; IV., membra- 

 nous roof of fourth 

 ventricle continuous 

 with the membra- 

 nous folds on each 

 side. Through these 

 the fimbrise (fb.) can 

 be dimly seen. 



