232 FREDERICK TILNEY AND LUTHER F. WARREN 



parietal eye or of a gland, or the simultaneous development of 

 both of these, is given in their origin. 



According to this conception, it is not possible to consider the 

 parietal eye as primordial; it seems far more likely that it is an 

 adaptive modification developing in response to special require- 

 ments in a limited number of forms. The proximal portion, on 

 the other hand, maintains its entity with such marked per- 

 sistency throughout the series that it seems possessed of the more 

 primitive characters. This is emphasized when the proximal 

 portion is considered in connection with the other glandular 

 derivatives of the diencephalic roof. Embryologically, in those 

 instances in which both an eye-like end- vesicle and a glandular 

 proximal portion develop the anlage of these parts must have 

 been pluripotential. This is equally true in the instances in 

 which one portion of the epiphyseal complex, as, for example, the 

 parapineal organ, develops an eye-like structure while the 

 pineal organ develops a marked tendency to glandular formation. 

 Such an interpretation of the pluripotentiality in the epiphyseal 

 anlage when applied to the various orders reveals the following 

 conditions : 



In cyclostomes the epiphyseal anlage seems to contain ele- 

 ments which are exclusively engaged in the differentiation of 

 eye-like structures which form the pineal and parapineal eyes. 



In selachians, ganoids, teleosts, and dipnoians the epiphyseal 

 anlage has completely lost its potentiality to differentiate as a 

 visual organ, and while there may be some debate as to the 

 character of the adult structures, there is some evidence which 

 points to their glandular nature. 



In amphibia both potentialities are present in the pineal 

 organ. In Sphenodon and lacertilia both potentialities are also 

 present, but in these instances the parapineal portion of the 

 epiphyseal complex gives rise to the eye-like structure while the 

 pineal portion develops glandular characters. In ophidians and 

 all the higher vertebrates the potentiality for the development 

 of visual structures is lost. 



Even accepting the probability of this dual potentiality, it 

 should be borne in mind that the median eye-like structure may 



