THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



299 



olfactory tracts. As indicated in Figures 8 B and 9, the great 

 growth of the front walls of the lateral lobes has resulted in their 

 apposed mesial faces fusing together so as to give the appearance 

 of an enormously thickened lamina terminalis. The preservation 

 of the anterior branch of the lateral ventricle and of the canal 

 from the dorsal surface serves to show the primary form of this 

 region. At either side of the recessus neuroporicus in Figure 8 



Lateral olfactory nucleus 



Epistriatum 

 Tr. olfacto-corticalis 



Tr. olf.-habenularis 



Tr. haben.-pedunc. 



N. terminalis 



Striatum 



Tr. olf.-hypothal. med. 



Tr. olf.- 

 hypothal. lat. 



| L. inferior 

 Tr. strio.-thalam. 



Fig. 147. A diagram of the fiber tracts in the forebrain of a selachian. The 

 mesial surface of the right half of the brain of Squalus acanthias is drawn and the 

 fiber tracts projected upon it. The course of the tracts is taken chiefly from the 

 description by Kappers, but the work of Catois, Edinger, Houser, Locy and others 

 has been considered. 



B is seen a ridge projecting into the lateral ventricle, the regio 

 untinata. This represents the front wall of the brain immediately 

 adjacent to the lamina terminalis. The great mass which has 

 been formed by the expansion, thickening and bending upward 

 of the front walls of the brain between the olfactory bulbs will 

 be called the mesial olfactory nucleus. 



The lateral walls have also grown up, thickened enormously 

 and fused together over the ventricle to form the massive roof 



