210 FREDERICK TILNEY AND LUTHER F. WARREN 



pineal organ manifests such marked alterations as to leave no 

 doubt that a process of specialization is going on in this struc- 

 ture. It is a much more voluminous organ having a greater 

 solidity and presenting only one of the three fundamental por- 

 tions observed in the pineal organ of the lower forms. The 

 end- vesicle and the stalk of the end-vesicle have disappeared. 

 The proximal portion alone remains to represent the epiphyseal 

 complex. It also manifests certain modifications in its relation 

 to the brain, since now it no longer communicates with the ven- 

 tricle through a canal. Furthermore, it has developed a shallow 

 intermediate stem or stalk connecting it with the -roof -plate. 



The inception of the process resulting in the formation of 

 the pineal peduncle is first witnessed in the sphenodon and 

 lacertilia. The conditions in birds and mammals show a still 

 further tendency along the lines of specialization first manifested 

 in ophidians, for, as in these latter forms, neither the parapineal 

 organ nor the end-vesicle or stalk of the pineal organ makes its 

 appearance. The epiphysis in many of the birds becomes a 

 solid organ with no canal connecting it with the third ventricle, 

 although in certain birds this canal is present. In mammals the 

 canal has never been observed and the pineal body presents 

 itself as a dense, solid structure in close proximity to the roof of 

 the interbrain or resting upon the roof-plate of the midbrain. 



Another observation made by Kidd 203 is pertinent in this con- 

 nection, to the effect that if nature is endeavoring to be rid of 

 the pineal body it has taken a remarkably long time imper- 

 fectly, if at all, to accomplish this end. The evidence that the 

 reptiles in the Palaeozoic era possessed a parietal eye is sub- 

 stantiated by the parietal foramen in these extinct forms, as 

 demonstrated by Bashford Dean. 82 



All of this evidence concerning the phyletic variations and 

 morphologic specialization seems to justify the conclusion that 

 the epiphyseal complex is possessed of a pluripotentiality which 

 in a few forms has been realized as a more or less diffuse visual 

 structure, but which fundamentally appears to be in the interest 

 of a differentiation whose functionl significance is not sensory. 



