THE PINEAL BODY 217 



1. The phyletic constancy of the epiphysis. 

 2. Its phyletic variations and morphologic specializations. 



3. Its relatively greater phyletic constancy with reference to 

 other structures in the pineal region. 



4. The phyletic predominance of the proximal portion of the 

 pineal organ. 



5. The evidence of its progressive specialization in ophidians, 

 birds, and mammals. 



6. The increase of the epiphyso-cerebral index froni the 

 earliest stages to the latest periods of life in man. 



7. The resistance to the encroachment of a prominent neo- 

 morph in the mammalian brain such as is the corpus callosum, 

 whose presence has produced such a marked alteration in the 

 other constitutents of the diencephalic roof-plate. 



3. Evidence based on the histology of the epiphyseal complex 



From the comparative histology of the epiphyseal complex, 

 it becomes evident that specialization in these organs has fol- 

 lowed two main lines: First, the structures have either differ- 

 entiated in the interest of forming visual organs or, second, 

 they have given rise to glandular itissue. In some instances, 

 both of these tendencies may be observed, that is to say, in 

 certain species the differentiation has been in the interest of 

 visual apparatus in one part of the epiphyseal complex, while in 

 another part, distinct glandular tendencies are apparent. It 

 seems advisable for the purpose of obtaining as comprehensive a 

 view as possible of the histology of this portion of the brain to 

 consider the leading features of the finer structure in the pineal 

 body of each of the classes of vertebrates. 



Histological evidence in cyclostomes. The striking histological 

 features in cyclostomes are the specializations in both pineal and 

 parapineal organs in the interest of forming visual structures. 

 The end-vesicle of the parapineal as well as the pineal organ 

 presents a retina. This structure in the pineal organ contains 

 cells of a distinct rod-like shape which have, therefore, been 

 designated the rod cells. Other cellular elements are also ob- 



