206 FREDERICK TILNEY AND LUTHER F. WARREN 



brates. This contention, while it must have its place in the 

 discussion, seems to lose force in view of the special development 

 of the pineal organ in certain vertebrates. Thus, in cyclostomes 

 there is present, to a degree seen in no other vertebrates, a 

 development of the major constituents of the epiphyseal com- 

 plex. That is to say, both the pineal organ and parapineal 

 organ attain a degree of differentiation which at least justifies 

 the supposition that one, if not both of them have functional 

 activities of a visual nature. The presence in these forms of a 

 well-marked retinal structure, seen in the pineal organ as well 

 as in the parapineal organ, an end-vesicle containing a syncytial 

 structure comparable in many respects to the vitreous, a pig- 

 ment-free, ectal wall enclosing the end-vesicle and resembling a 

 lens, together with a bundle of nerve fibers connected with the 

 posterior commissure in the case of the pineal organ, and the 

 superior commissure . in the case of the parapineal organ, con- 

 stitute irrefutable evidence of morphological specialization 

 adapting the organ to photo-receptive, if not visual purposes. 

 This supposition is further borne out by the fact that the organs 

 in their development grow rapidly away from the roof of the 

 brain and ultimately take up a position which, from its relation 

 to the surface of the body, affords certain epiphyseal structures 

 the best opportunity of becoming distance receptors. From the 

 striking position which the pineal and parapineal organs hold in 

 the vault of the skull, lodged as they are in a deep fossa, it would 

 seem evident that they have become so situated that they might 

 the more readily receive sensory impulses impinging upon the 

 surface of the head. 



That this visual or photo-receptive tendency in the selachians, 

 ganoids, and teleosts should almost altogether disappear, al- 

 though the pineal organ itself remains as a conspicuous struc- 

 ture, would speak in favor of a pluripotentiality in the differen- 

 tiation of the epiphyseal complex. It is certain that in the 

 higher fish there is no evidence pointing to the development of 

 anything resembling the visual structures observed in cyclo- 

 stomes. In selachians the parapineal organ is entirely absent; 

 the pineal organ, en the other hand, is a large and prominent 



