202 FREDERICK TILNEY AND LUTHER F. WARREN 



1 . Pineal organ. 2. Parapineal organ: 



a) Proximal portion. a) Proximal portion. 



6) Stalk. 6) Stalk. 



c) End-vesicle. c) End-vesicle. 



Of all of these parts the proximal portion of the pineal organ 

 is phyletically the most constant, occurring in all classes of 

 vertebrates. The stalk and end- vesicle of the pineal organ 

 are much less constant, for they cease to appear in ophidians 

 and are absent in all the forms higher than the snakes. The 

 parapineal organ as a whole is limited to but a few classes of 

 vertebrates. It is prominent only in cyclostomes, in prosaurians 

 and saurians. It is rudimentary in ganoids and teleosts. It is 

 developmentally transitory or entirely absent in selachians, 

 amphibia, ophidians, chelonia, crocodilians, birds, and mammals. 



Considered, for a moment, quite apart from the inferences 

 which may be drawn from the intrinsic structural characters of 

 the epiphyseal complex itself, there is one outstanding feature 

 of unquestionable importance, namely, the genetic association of 

 this complex with a series of organs which under no conditions 

 have manifested a tendency to become specialized in the interest 

 of definitive neural mechanisms, but which, wherever differ- 

 entiated, have given rise to glandular tissue. 



The caudalmost element in the pineal region is the posterior 

 commissure. It is, perhaps, not definitely settled that this 

 assignment of the commissure to the interbrain is in all respects 

 justifiable. If, however, it is to be accounted as a structure of 

 the pineal region, the function of the commissure appears to be 

 related to a specialized portion of the pineal organ, namely, the 

 end-vesicle, with which latter the posterior commissure is said to 

 be in connection by means of nerve fibers. Admitting, for the 

 moment, the correctness of the morphological and physiological 

 interpretation given the subcommissural body by Dendy and 

 Nicolls, 88 the structure may tentatively be considered as a 

 part of the pineal region. Its function, apparently, is in some 

 way connected with the fiber of Reissner and the entire organ 

 thus associated with equilibration. Both of these elements, 



