434 C. JUDSON HEKRICK 



hemispheres. The true hemispheres, as the diagram (fig. 6) 

 shows, are represented only by the evaginated olfactory bulbs. 

 The thickenings of the primitive endbrain of teleosts have been 

 described as everted in contrast with the extensively evaginated 

 hemispheres of Amphibia, but this is hardly an adequate state- 

 ment of the case. It is no doubt true that there has been some 

 lateral eversion of the wall analogous with that so strikingly 

 shown by Polypterus (fig. 11). But Sheldon ('12) has shown 

 that the teleostean form has not been reached by a simple bending 

 of the massive lateral wall outward, as is clearly the case in 



Fig. 8 Diagram of the relations of the walls of the f orebrain and its ventricles 

 in the dogfish, Squalus acanthias. The line A-B indicates the plane of section 

 of figure 9; C~D that of figure 10. 



The relations of these ventricles are quite different from those of Mustelus 

 canis, as figured by Johnston ('06, fig. 8), but they conform closely to his diagram 

 ('06, fig. 9) "to show what is believed to be the primitive relations of the wall 

 of the ventricle." 



Polypterus, but chiefly by the accumulation of additional nervous 

 tissue in the middle of the wall with more or less plastic rearrange- 

 ment of the component nerve centers. 



The elasmobranchs, as illustrated by the dogfish, Squalus 

 acanthias (figs. 8, 9, and 10), show a different arrangement of 

 the correlation centers at the bases of the widely evaginated 

 olfactory bulbs. The thickening here takes place chiefly in and 

 adjacent to the terminal plate, a considerable part of the thick- 

 ened tissues lying above the ventricle and hence not shown in 

 figure 8 (cf. figs. 9 and 10). At the base of the olfactory bulb 



