ORIGIN OF THE CEREBRAL HEMISPHERES 



433 



The bony ganoids, as illustrated by Amia, and their alhes, 

 the teleosts, have gone to the hmits of extreme speciahzation in 

 this direction. In late embryonic stages the condition is sub- 

 stantially as figured for the sturgeon, but in the adults of some 



COMMON VENTRICLE 



PRIMITIVE ENOBRAIN 



Fig. 4 Diagrammatic longitudinal section of the forebrain of the sturgeon, 

 Acipenser. The cerebral hemispheres comprise only the olfactory bulbs, the 

 remainder of the endbrain being unevaginated, but thickened laterally. 



Fig. 5 Cross-section through the middle of the thickened primitive endbrain 

 of the sturgeon. Figures 4 and 5 are based on the figures of Johnston ('01). 



Fig. 6 Diagrammatic longitudinal section through the forebrain of a teleost. 

 As in the sturgeon, the true, evaginated cerebral hemispheres contain only the 

 olfactory bulbs, below which there are great thickenings of the unevaginated 

 primitive endbrain, which arise first embryologically in the side walls, as in the 

 sturgeon, but in the adult are attached only to the floor. 



Fig. 7 Cross-section through the thickened primitive endbrain of a teleost. 



of the species the lateral thickenings have enormously increased 

 so that they bulge upward into the common ventricle and are 

 attached only on the ventral side, as illustrated in figures 6 and 

 7. These thickenings are sometimes infelicitously called cerebral 



