Johnston, Forehrain Vesicle in Vertebrates. 



465 



chorioideus of the telencephalon. The anterior limb of the plexus 

 is attached to the massive nervous wall which overarches the front 

 part of the median ventricle. In the more primitive selachians such 

 as Heptanchus and in Chimsera the massive roof is smaller and the 

 membraneous roof extends farther forward. As shown in Fiff. 1, 

 the massive roof is pierced from the dors'ocephalic surface by a vas- 

 cular canal which reaches nearly to the ventricle. This has been 

 interpreted (Johnston, 1906) as the remnant of a deep groove which 

 separated the lateral lobes of the telencephalon earlier in the phylo- 



IV vent 



Fig. 2. A, an outline of the brain and brain ventricles of Mustelus as 

 seen from above. B, a diagram of one side of the fore-brain to show the 

 primitive relations of the wall and ventricle. From Johnston, 1906. 



geny of selachians, and the process by which the present fonn was 

 reached is indicated by the accompanying diagram (Fig. 2, B). 



That portion of the massive roof which lies behind the vascular 

 canal is formed by the great growth of the lateral walls and it has 

 for its basis a fiber-decussation which is comparable in position and 

 in a part of its fiber components to the superior olfactory decussation 

 of Petromyzon. In selachians, then, owing to the enormous devel- 

 opment of the olfactory centers, that part of the membraneous roof of 

 the telencephalon which contains fibers in cyclostomes is invaded by 

 gray matter as well. 



