208 FREDERICK TILNEY AND LUTHER F. WARREN 



reaches dimensions comparable with the pineal organ. As in 

 the case of the selachians, the histology of the pineal organ of 

 the teleost will prove helpful in the interpretation of its function. 



Thus, in the four great classes of fish, cyclostomes, selachians, 

 ganoids, and teleosts, the epiphyseal complex shows a remark- 

 able variation in its specialization, and while the tendency to 

 develop visual or photo-receptive structures is marked in the 

 cyclostomes, it is suspended, if not entirely absent, in selachians, 

 ganoids, and teleosts. 



In the latter forms, however, the epiphyseal complex is so 

 conspicuous an element of the brain as to make the conclusion 

 that it is without function a difficult one to maintain. For these 

 reasons, it would seem justifiable to conclude that the epiphyseal 

 complex is pluripotential in its specialization and that while it 

 may be vested with the possibility of giving rise to a visual or 

 photo-receptive apparatus, it may and does become differ- 

 entiated as organs having some significance other than sensory. 



Still greater modifications present themselves in the amphibia, 

 for in these forms the pineal organ shows an even more marked 

 differentiation than any of the other lower classes. The para- 

 pineal organ does not develop in urodela or anura. When, how- 

 ever, the structure of the pineal organ is considered, the fact 

 that it develops a proximal portion of such conspicuous dimen- 

 sions as to be secondary to the paraphysis in the roof of the 

 interbrain, from the free extremity of which there extends a thin 

 nerve filament connecting with an end-vesicle, it becomes clear 

 that the entire process of adaptation in this instance cannot be 

 in the interest of sensory function, for if that were the case, 

 why, then, should the proximal portion of the pineal organ 

 assume such conspicuity? 



Stieda's 379 interpretation of the end- vesicle in amphibia as a 

 frontal subcutaneous gland is, of course, quite untenable, since 

 the end-vesicle manifests by its position and connections some 

 obvious adaptation to sensory activity. Yet to its proximal 

 portion might well be attributed a glandular function, not 

 only because of its unusually large dimensions, but also because 

 of the position which it holds with reference to the third ventricle. 



