222 FREDERICK TILNEY AND LUTHER F. WARREN 



fossa, and parietal spot, all give evidence of the most complete 

 adaptation for visual function. 



Studnicka 391 believes that the rich capillary blood supply in 

 ophidia speaks in favor of the glandular nature of the organ, 

 its secretion being contributed to the blood stream. In chelonia 

 the cellular elements are mostly ependymal and neurogliar and 

 no nerve cells or nerve elements are found. There is, however, 

 no clear evidence of the secretory nature of the epiphyseal com-' 

 plex in these forms. 



The conclusions which may be drawn with reference to reptiles 

 seem to indicate that in the primitive forms the parapineal organ 

 assumes the highest differentiation which it attains as a visual 

 structure. There is some evidence that the pineal organ, even 

 in the animals, manifests a tendency toward glandular forma- 

 tion. In ophidians, however, there can scarcely be a doubt 

 that the proximal portion of the pineal organ is the only element 

 which persists and that it has a definitely glandular structure. 

 This is probably true also in chelonians. The pineal gland in the 

 snake and turtle probably contributes its secretion to the blood 

 stream, but may also impart a portion of it to the cerebrospinal 

 fluid. The more recent reptiles manifest no disposition on the 

 part of the epiphyseal complex to develop any sensory or other 

 type of neural mechanism. 



Histological evidence in birds. The conspicuous change in the 

 epiphyseal complex noted in the transition from the primitive 

 reptiles to those of more recent history is strikingly emphasized 

 when the condition's in this region of the brain in birds are 

 reviewed. Here, as in the snakes and turtles, there is com- 

 plete suppression of the parapineal organ, and that tendency 

 toward the differentiation of a visual apparatus which seems to 

 have reached its height in Sphenodon, has so far receded as to 

 leave no indication in birds of its earlier existence. This histo- 

 logical feature of itself is highly significant, but when taken in 

 conjunction with the appearance offered by the finer structure of 

 the pineal body in birds, it seems to set all doubt aside as to the 

 inherent tendency of the epiphyseal complex along its major 

 lines of differentiation. In every species of birds which has so 



