THE PINEAL BODY 227 



has not always been restricted to the sense here advocated, and 

 yet, as has been previously pointed out, it was from precisely 

 the conditions in mammals that the descriptive conception, 

 pineal body, took origin. 



The theory that the pineal body is the vestige of the parietal 

 eye is accepted by many. According to this view, the third 

 eye of vertebrates should be regarded as primordial and the 

 pineal body an arrested development in the attempt to reach 

 such differentiation. The evidence, however, is by no means 

 conclusive, for, as has previously been shown, the entire epi- 

 physeal complex springs from a region which is fundamentally 

 glandiferous, while only in a very few instances is a tendency 

 toward sensory differentiation recognizable in it. By far the 

 great majority of vertebrates manifest in the epiphyseal complex 

 no tendency whatsoever toward the development of any neural 

 mechanism. This would seem to indicate that the tendency for 

 the epiphyseal complex to develop visual structures is a secon- 

 dary and not a primordial character. Furthermore, if the 

 pineal body was in any true sense the vestige of the parietal 

 eye, it would seem almost inevitable that the organ should con- 

 tain remnants indicative of visual specialization. The absence 

 of such evidence at least raises a reasonable doubt that the pineal 

 body had at any time possessed visual function. The almost 

 universal absence of true ganglionic cells as well as the lack of 

 nerve fibers, which may be regarded as belonging to some cate- 

 gory other than those of the sympathetic system, would seem to 

 call into question the possibility of the pineal body ever having 

 participated in the formation of a neural mechanism. This may 

 be considered negative evidence. There remains to be men- 

 tioned, however, the significant fact that the pineal body in all 

 of the higher vertebrates manifests a tendency to differentiate 

 along lines which cannot be interpreted as in the interests of 

 visual function. As has been previously shown, the differen- 

 tiation which does occur in the higher reptiles, birds, and mam- 

 mals gives rise to glandular tissue, ^rom these facts it seems 

 possible to conclude that the pineal body is not a vestige of the 

 parietal eye. 



