THE PINEAL BODY 207 



structure presenting a proximal portion, a prolonged stalk, and 

 a fairly well marked end-vesicle. No tendency, however, is 

 observed toward the development of a photo-receptive appara- 

 tus. The thickened proximal portion communicates directly 

 with the ventricle on the one hand, and through the stalk with 

 the end-vesicle on the other. The fact that this latter portion 

 of the pineal organ is lodged in a deep fossa of the skull and thus 

 brought into close relation with the epidermis, would favor the 

 belief that in this structure may be observed the arrested or 

 abortive effort toward the formation of a physiological organ. 

 That this organ, however, deprived of the opportunity to reach 

 such a goal in its differentiation, should remain so prominent a 

 structure connected with the brain, would seem to refute the 

 conception that it is a mere vestige or rudiment; indeed, it 

 seems to compel the belief that it exists in the interest of some 

 other function as yet not entirely clear. 



When, however, the finer histological structure of the pineal 

 organ in selachians is discussed, it may be possible to disclose 

 evidence which will at least suggest, that the structure in these 

 forms is functionally active. The point which the gross mor- 

 phological conditions in selachians does lay emphasis upon is the 

 presence of so prominent a structure, showing no evidence in 

 itself of retrogression and yet quite devoid of such specializa- 

 tion as would connect it definitely with visual function. 



In the teleosts, the observation made with reference to the 

 selachians assumes even more importance, for here the pineal 

 organ shows a marked specialization which is entirely contrary 

 to the lines of differentiation followed in the development of a 

 visual organ. In most of the teleosts the pineal organ presents 

 a small proximal portion, a relatively short stalk, and a volumi- 

 nous thick-walled end-vesicle. The general follicular appear- 

 ance of the end-vesicle, together with its relatively large size 

 and the fact that it has neither migrated to such a great distance 

 from the roof-plate of the interbrain nor come to occupy a defi- 

 nite fossa in the vault of the skull, all go to disprove any inherent 

 tendency in the structure to differentiate as a visual organ. In 

 many teleosts a small parapineal organ develops, but never 



