446 



all show the fibres iu essentially the same condition, Fig. 2. There 

 is then no doubt of its being a normal structure, in the brains of 

 these primitive vertebrates. 



An examination of the brain of both larval and adult Petromyzon 

 failed to show its presence in these forms, but in its place a fine- 

 meshed net-work of fibrils was found which bears the same relation 

 to the ependyma cells and in life, practically fills the ventricular cavity. 



Fig. 2. A portion of the same, more highly magnified, to show the course of the 

 central coils of the fibre, and its branches, extending out into the ventricular sijaces. 



In Fig. 3 is shown three large branches of the fibre issuing from 

 the cerebellar and mid- brain ventricles on their way to join the main 

 fibre further down, as shown in Figs. 1 and 2. 



The origin of the ultimate fibrils is shown in Fig. 4. The pro- 

 cesses from the ependyma cells unite to form a net work of fibrils, 

 comparable with that found in the brain of Petromyzon, but out of 

 this net work, issue large trunks which unite one after another into 

 still larger fibres until all are brought together in the coiled fibre of 

 the largest chamber of the ventricular cavity. 



