THE PINEAL BODY 231 



end-vesicle and the proximal portion of the pineal organ. Were 

 it not for a slender fasciculus of nerve fibers these two portions 

 of the epiphyseal complex would appear as independent entities. 

 As it is, both parts are well differentiated and well developed, 

 one as an eye-like organ, the other with some of the characters 

 of a gland. This distinction between the end-vesicle and prox- 

 imal portion should not be underestimated. It not only shows 

 how remote the relationship between the two parts may be, 

 but also gives an added prominence to the proximal portion. 

 This latter part has already been shown to be the most constant 

 element in the epiphyseal complex, while the end-vesicle is much 

 more limited in its occurrence. 



The process by means of which the end-vesicle and proximal 

 portion of the pineal organ are rendered so distinctive in amphibia 

 takes on a new phase in Sphenodon and lacertilia. In these 

 forms the necessity for the end-vesicle to assume visual char- 

 acters has apparently ceased, and this structure together with 

 the stalk is evidently in a state of involution. The contrary, 

 however, is true of the proximal portion which has taken on 

 not only more conspicuous dimensions, but also more pro- 

 nounced glandular characters. In the ophidians, in birds, and 

 in mammals the process of involution in the end-vesicle and 

 stalk has been carried to its final stage. No trace of the end- 

 vesicle or the stalk is to be found in any of the orders above 

 lacertilia. The proximal portion, on the other hand, in ophidi- 

 ans, birds, and mammals gains prominence because of its 

 glandular structure. 



The process here described from amphibia to mammals clearly 

 demonstrates the progressive involution of the eye-like end- 

 vesicle and the gradual ascendency of the glandular proximal 

 portion. At one end, namely in amphibia, the end-vesicle and 

 proximal portion must be regarded as coordinate in prominence. 

 At the other end, i.e., in ophidia, the proximal portion is pre- 

 eminent because of the disappearance of the end-vesicle. This 

 phenomenon can best be interpreted on the basis of a pluri- 

 potentiality in the anlage of the epiphyseal complex, of such a 

 nature that the adaptive possibility for the development of a 



