MUSEUM OF COMPARATIVE ZOOLOGY. 223 



It seems to me that Leydig has not given sufficient prominence to 

 the possibility, not to say great probability, that the nervous connection 

 has been lost by the modification and degeneration which the whole 

 structure has certainly undergone ; and especially must we hesitate in 

 rejecting this explanation, when we remember that by so doing we are 

 compelled to seek another. To be obliged to ascribe a function other 

 than that of vision to a structure entirely like an organ of vision in 

 most of its essential parts, and differing widely from one in no essential 

 point, is requiring us to accept a conclusion that would throw suspicion 

 on all our morphological reasoning. Should it be shown conclusively 

 that the vesicle never has, in any vertebrate, either in the adult or dur- 

 ing its ontogeny, nervous connection with the brain, then we should be 

 obliged to abandon the optical explanation of its origin, and turn to 

 the exceedingly difficult task of finding another. But until such 

 knowledge is at hand, it seems to me we must suppose that the organ 

 was produced as an eye, that in some way entirely unknown to us it 

 lost its optical function, and that, in the consequent modification and 

 degeneration, the optic nerve degenerated more rapidly in some cases 

 than did the optic vesicle ; and that in this way the separation which 

 we now find took place. 1 



In .previous discussions of the nature and function of the parietal 

 organ, I believe sufficient attention has not been given to the structure 

 and development of the epiphysis and its relation to the parietal ves- 

 icle, and especially its relation to the so-called choroid plexus. I have 

 designated the entire structure found in connection with the roof of the 

 thalamencephalon as the epiphysis ; but, as already said, I have consid- 

 erable doubt as to the wisdom of so doing. For the sake of precision it 

 would seem best that the term epiphysis should be limited to the 

 structure which arises as an evagination from this portion of the brain. 

 Certain it is that the large blood sinus which I have described as a part 

 of the epiphysis in Phrynosoma cannot be regarded as forming an essen- 

 tial portion of the structure, and I think it quite possible that what I 

 have called the epiphysial vesicle is not a portion of the epiphysis, should 



1 Concerning the nervous connection between the eye and the epiphysis in 

 Anguis fragilis, Stralil and Martin say ('88, p. 154), " Der Nerv der nach hinten 

 am Vorderrand der Epiphyse seheinbar verschwindet, tritt von unten her in das 

 Auge ein." Francotte ('88, p. 782) also describes essentially the same condition 

 in this species. But such a condition would be so anomalous that C. K. Hoffmann 

 ('88, p. 1991), notwithstanding the agreement of these independent statements, 

 has, it seems to me with reason, expressed doubt as to the trustworthiness of the 

 observations. 



