chap. xii. i BRAINS OF FISHES AND AMPHIBIA. 421 



wal) between the two parts of the fore-brain becomes 

 thinner, and a " primitive cerebral fissure " is apparent. 

 This condition of things is retained by some Ganoids 

 throughout life (Polypterus ; Fig. 180); these fishes 

 also possess the more primitive character of a large 

 thalamencephalon. 



In the Teleosfei the brain is compressed, the 

 cerebral hemispheres are almost completely solid, and 

 the cerebellum is usually, though not always, of com- 

 paratively large size ; it is often prolonged into the 

 cavity of the mid-brain (valvula cerebelli) ; on the 



Fig. ISO. Brain of Polypteriis seeu from the Side, 



t Olfactory nerves ; h, k, cerebral hemispheres o, optic nerve ; d, optic lobes ; 

 e, hypophysis ;/ r central fissure ; 6, c, cerebellum; a, mednlla oblongata. (After 

 J. Miiller.) 



whole, the brain of the Teleostei exhibits many resem- 

 blances to that of Ganoids, and especially of Lepi- 

 closteus. 



In the adult Amphibia, as in the adults of most 

 fishes, the several parts of the brain lie in the same 

 plane ; 011 the whole, the brain of the Anura is more 

 highly organised than that of the Urodela ; it is pro- 

 portionately larger than that of fishes, but is still 

 small. The brain of the Anura is different from 

 that of all other Vertebrates, owing to the fact that 

 the olfactory lobes of the adult are not separated 

 from one another, and, like that of the Urodela, the 

 cerebellum of the Anura is of extraordinarily small 

 size. 



