374 COMPARATIVE ZOOLOGY OF CHORDATES 



component-system), and where adjustments are made with 

 association and efferent neurons so as to complete the reflex 

 circuit. 



Now if these primary centres are marked off in the brain 

 of a dogfish, it is found that except for the cerebellum, they 

 occupy nearly the whole of the brain. Those regions of the 

 brain which conform to the organisation of the spinal cord are 

 called the " segmental apparatus " or " brain-stem," and are 



Fig. 171. — Transverse sections through the end-brains of, A, dogfish ; B, 

 frog ; C, Chelonian (reptile) ; and D, shrew (mammal). 



Showing the development of the cerebral hemispheres and lateral 

 ventricles, and the migration of nerve-cells to the surface forming a cortex. 



to be distinguished from the additions in the shape of the 

 cerebellum, and in higher forms the cerebral cortex, which are 

 " suprasegmental " structures. 



The various centres of the brain of the fish are mainly 

 concerned with their own functional component-system ; there 

 is not much " team work " between the different centres. 

 The result is that the behaviour of fish largely takes the form 

 of reflex responses to stimuli of certain kinds without much 



