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FREDERICK TILNEY AND LUTHER F. WARREN 



extent, although in certain cases it becomes much attenuated 

 as it approaches the end-vesicle. A few nerve fibers course in 

 the dorsal wall of this hollow stalk, but these cannot properly be 

 considered the homologue of the pineal nerve in selachians. 



The proximal portion in selachians may be readily made out. 

 As the stalk approaches the roof of the interbrain, it gradually 

 becomes dilated and increased in its transverse diameter. Its 

 lumen becomes larger and the walls bounding it are thrown into 

 numerous folds. Although the transition from stalk to proximal 

 portion is gradual, it is nevertheless distinct. In a few cases 



Fig. 49 End-vesicle in the pineal organ of Acanthias vulgaris, according to 

 Studnicka, 1893. 



only, such, for example, as Centrophorus, described by Cattie 60 

 in 1882, is there an absence of this reduplication of the walls of 

 the proximal portion. As the dorsal wall of this portion ap- 

 proaches the posterior commissure there appear in it a few 

 strands of nerve fibers constituting what may be called the 

 tractus pinealis. It is doubtful, however, whether the com- 

 missura habenularis receives any of the fibers which enter into 

 the formation of this tract. 



The sheaths of the pineal organ are the same as those in Petro- 

 myzon, namely, a membrana limitans externa, a process from 



