ORIGIN OF THE CEREBRAL HEMISPHERES 435 



there is a considerable amount of evaginated tissue which is 

 continuous with the unevaginated primitive endbrain behind 

 the wide interventricular foramen. The hemisphere, therefore, 

 contains the fully evaginated olfactory bulb and some adjacent 

 tissue which is not sharply marked off from that of the unevagi- 

 nated primitive endbrain. 



Both elasmobranchs and teleosts are very efficient types of 

 fishes. Biologically they dominate their respective environ- 

 ments. This dominant position, however, has been brought 

 about by very precise adaptation of each individual species to a 

 particular habitat and mode of life on the reflex plane, rather 



Fig. 9 Cross-section through the primitive endbrain and common ventricle 

 of Squalus acanthias in the plane A-B of figure 8. The lateral dilations of the 

 common ventricle are continued forward into the lateral ventricles; the large 

 masses above the ventricle are continued forward into the cerebral hemispheres 

 (cf. fig. 10). Modified from Johnston ('11, fig. 54). 



Fig. 10 Cross-section through the cerebral hemispheres of Squalus acanthias 

 in the plane C-D of figure 8. Modified from Johnston ('11, fig. 57). 



than by an increase in capacity for individual adjustment to 

 diverse and variable conditions. And neither group has given 

 rise to anything higher. They form terminal branches of the 

 phylogenetic tree. These types of forebrain appear to be incap- 

 able of expansion in directions affording greater flexibility and 

 modifiabihty of behavior of the individual. 



The forebrain of Polypterus, the most primitive of existing 

 ganoids, is in some respects very close to the architypical form 

 illustrated in figure 3, but in other respects it is aberrant. Poly- 

 pterus is unique among living fishes, not only in forebrain struc- 

 ture, but in other respects also. By the systematists it is widely 

 separated from the other members of the old group of so-caUed 



