THE PINEAL BODY 209 



If the argument bearing upon the pluripotentiality of specializa- 

 tion of the epiphyseal complex needed support or confirmation,, 

 this is found in the conditions of amphibians. 



The evidence afforded by the reptiles goes, perhaps, as far 

 as may be deemed necessary to confirm the pluripotentiality of 

 the pineal organ and its derivatives. In the ancient and primi- 

 tive reptiles, including the prosaurians and saurians, there is a 

 tendency for both pineal and parapineal organs to attain remark- 

 able development. But in these forms, it is the parapineal organ 

 which assumes predominance in the development of a sensory 

 apparatus. In sphenodon and many of the lizards the parietal 

 or third eye reaches such a high state of differentiation as to 

 leave little doubt concerning its visual function. The well 

 marked optic vesicle, lodged in a parietal fossa and brought into 

 relation with the external epidermis by means of specialized cells, 

 affords incontrovertible evidence that this organ is adapted as a 

 distance receptor. The pineal organ, while it presents some 

 tendency towards the development of a visual organ, does, as a 

 matter of fact, fall far short of such attainment. Its end-vesicle 

 is smaller than in any of the other forms already considered. 

 Its stalk is shorter; on the other hand, its proximal portion has 

 assumed characters not as yet observed in the lower members of 

 the vertebrate series. So pronounced is the specialization of 

 this proximal portion that it needs no microscopic investigation 

 to disclose the marked differentiation of the structure. Its 

 walls are not only thick and convoluted, giving it a lobulated 

 appearance, but its diameters are greater than those of the 

 lower forms. 



Upon passing to the more modern reptiles, including the 

 ophidians and chelonians, the tendency to specialization which 

 has previously been emphasized in this discussion, receives still 

 further accentuation. In these forms, the parapineal organ dis- 

 appears altogether and nothing remains to indicate that it ever 

 had existence in reptilia. There is no parietal fossa and no 

 specialization of the cutaneous surface in the head which might 

 even vaguely suggest the remnants of the parietal eye so con- 

 spicuous in the ancient reptiles. Yet, on the other hand, the 



MEMOIE NO. 9 



