Eycleshymer-Davis, Epiphysis and Paraphysis in Amia. 6"/ 



isfactory nature of the figure (Fig. 3a) in which the lumina 

 might well be interpreted as mere folds in the brain wall, render 

 the evidence on this point extremely unconvincing, moreover 

 the usual condition observed by the author in Salmo is that 

 shown in Fig. 4. 



Again, the disposition of the two vesicles in Catostomus 

 and Stizostedion as shown in Figs. 5 and 8 certainly accords 

 with the view that the anterior is derived from the posterior. 



Hill states that " The only reason for regarding the anterior 

 vesicle as formed at the expe?ise of the distal end of the posterior ves- 

 icle is that it is smaller, and, aside from this single fact, one might 

 with equal force consider the posterior vesicle as formed at the ex- 

 pense of the anterior.'' Since Hill does not in the present paper 

 describe the origin of either vesicle in a single form it is evi- 

 dent that he has in mind a condition in which the vesicles are both 

 well established. The above statement when applied to such a 

 stage is undoubtedly correct, but if we turn to Hill's descrip- 

 tion of Coregonus as given in an earlier paper we read : ' ' The 

 posterior vesicle appeared about two days before the anterior." 



Our observations on Amia show that the posterior is well 

 differentiated long before the anterior appears. These facts 

 would seem to show that in these forms the anterior could 

 not be considered as giving rise to the posterior, and we venture 

 to predict that when the earlier development is made known in 

 the other forms studied by Hill the time relations existing will ne- 

 cessitate a revision of the above statement for these forms as 

 well. 



"If we regard the position in Salmo as the primitive one, 

 some shifting must have taken place in the other forms. In 

 this shifting the determining factor appears to have been the 

 degree of development of the vesicles. Thus the relatively 

 large size of the anterior vesicle of Lacertilia, due to its former 

 functional importance, may have brought it into the median 

 plane, because there was room for it only in that position. The 

 epiphysis in Lacertilia may have come to lie in the median plane 

 for the same reason. In Teleosts on the other hand, the 

 epiphysis alone becomes of considerable size, and the small an- 



