THE PINEAL BODY 233 



in no instance signify a functionally active visual organ. In all 

 cases the attempt to develop a median eye may represent but 

 the abortive and partially attained differentiation of far remote 

 primitive ancestors in which such an eye was functionally active. 

 Its persistence into extant forms even as an abortive structure 

 may thus be taken to indicate the transmitted potentiality of 

 the epiphyseal complex to develop a visual organ. 



The theory that the two elements in the epiphyseal complex, 

 namely, the pineal and parapineal organs, represent a pair of 

 parietal eyes similar to those of invertebrates, has little to recom- 

 mend it. The hypothesis of Dendy 86 that the ancestral verte- 

 brates were possessed of. such a pair of visual organs, while 

 interesting, is based upon too few facts in living vertebrates to 

 justify its acceptance. After considering the several theories 

 concerning the relation of the parietal eye to the pineal body, we 

 have come to the conclusion that none of them is adequate to 

 explain all of the facts. But with a full appreciation of the 

 investigation already devoted to this subject we desire to offer 

 a new interpretation which to us seems more tenable. Accord- 

 rag to our views, there is no direct relation between the parietal 

 eye and the pineal body, but each is of itself an adaptive modifi- 

 cation answering the demands for, or representing an inherent 

 impulse toward, the development of a parietal eye, on the one 

 hand, or of a glandular organ, on the other. In other words, the 

 epiphyseal anlage is pluripotential in its derivatives. 



5. The phylogenetic significance of the parietal eye with reference 

 to vertebrates and invertebrates 



Much has been written concerning the significance of the 

 parietal eye as one of the possible indices in the evolution from 

 invertebrates to vertebrates. Although little evidence bearing 

 upon this point has been presented in the general consideration 

 of this work, the subject seems of enough interest to warrant the 

 inclusion of the views of certain investigators who have devoted 

 some attention to this matter. 



