ii BRAIN 95 



has probably been linked up more particularly with the processes of 

 localization or centralization of particular functions in particular 

 brain regions. When this has come about, increase in the number 

 of ganglion-cells devoted to the particular function will cause an 

 increase in bulk of that portion of the brain in which they are situated 

 and it will assume definite characteristics of its own. 



The first step in the development of such a brain region consists 

 in the mere thickening of the brain wall but with still greater 

 increase in the number of cellular elements involved mere increase 

 in thickness becomes insufficient for their accommodation and an 

 increase in area comes about in addition. This necessarily causes a 

 bulging of the particular part of the brain wall and some of the most 

 characteristic differences between the brains of different types of 

 Vertebrate depend upon whether the bulging takes place outwards 

 or inwards. 



Thus in the majority of Vertebrates the cerebellum bulges out- 

 wards as has been indicated in the case of Acanthias. In Teleostean 

 fishes on the other hand this is the case with only the hinder part of 

 the cerebellum : its anterior portion in these fishes bulges downwards 

 and forwards underneath the roof of the mesencephalon forming the 

 well-known valvula cerebelli. In the more primitive ganoid fishes 

 on the other hand such as Polypterus (Graham Kerr, 1907) the hind , 

 portion of the cerebellum also grows inwards, so as to form a structure 

 projecting back into the fourth ventricle in just the same fashion as 

 the valvula cerebelli projects forwards. 



A somewhat similar difference appears to be present in the case 

 of the hemispheres. These originate in most subdivisions of the 

 Vertebrata as paired bulgings of the wall of the primitive fore-brain, 

 and the present writer agrees with Studnicka (1896) in feeling com- 

 pelled to accept on this ground the view taught by many of the older 

 morphologists such as von Baer, Beichert and Goethe that the hemi- 

 spheres are to be looked on as fundamentally paired structures, 

 rather than the view, more fashionable of recent years, which 

 regards the portion of the primitive brain lying in front of the 

 velum and optic recess as forming together with the hemisphere 

 region an unpaired complex (Telencephalon — His). The more 

 complete knowledge that we now possess regarding the develop- 

 ment of the brain in the more primitive Vertebrates with holo- 

 blastic eggs, seems to the writer to make it clear that the reasons 

 which have led to a departure from the older view can no longer be 

 regarded as adequate. We take it then that the hemispheres are 

 fundamentally paired projections of the side wall of the primitive 

 fore-brain. Physiologically they are probably to be regarded as 

 portions of the brain wall which have become specially enlarged in 

 relation with the sense of smell, just as are the optic outgrowths in 

 relation with the sense of sight. 



Now whereas in the majority of Vertebrates the hemispheres 

 bulge outwards, in the more primitive Teleostomes (e.g. Polypterus, 



