226 FREDERICK TILNEY AND LUTHER F. WARREN 



attributed to neuroglia in the ordinary sense and most in favor 

 of a glandular activity. 



The observations of Nicolas, 283A later confirmed by Dimitrova, 92 

 in which muscle cells were reported as histological elements of 

 the epiphysis in several Ungulates, have not been confirmed by 

 any other observers, and some authorities have been categorical 

 in their affirmation concerning the absence of such elements. 

 That the epiphysis may contain nerve cells and nerve fibers is 

 probable, but there is no evidence in mammals of the existence 

 of any neural mechanism in the pineal body. 



To consider the epiphysis in mammalia as a vestige in the 

 light of the histological evidence here summarized seems to be 

 an attitude which is wholly untenable, all the more so when 

 this histological evidence points to the fact that the structure 

 is a gland. For in this respect not only is the character of the 

 cells significant, but their arrangement in definite acini, the rich 

 vascular network about these acini, and the trabeculation by 

 means of connective tissue which gives this structure the appear- 

 ance common to all glands, are also suggestive of this fact. 



The final conclusion to be drawn from the histological evidence 

 in the epiphyseal complex of vertebrates would seem clearly to 

 indicate that this structure of the pineal region possesses a 

 pluripotentiality whose fundamental, inherent tendency is in 

 the interest of glandular differentiation and that in a few in- 

 stances, as in cyclostomes, amphibia, and in primitive reptiles, 

 the parapineal or pineal organ may become further differentiated 

 in the interest of a highly specialized sensory mechanism which 

 has, or has had, visual function. 



4. The relation of the parietal eye to the pineal body 



Much of the difficulty in interpreting the relation between 

 the parietal eye and pineal body arises from a confusion in the 

 use of terms. If by pineal body is meant the epiphysis as it 

 appears in mammals, it becomes relatively simple to discuss 

 the relation between this structure and the third eye of verte- 

 brates. It may perhaps be arbitrary thus to limit a term which 



