116 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



cephalad and ventrad by the progressive fusion of the edges of the 

 groove. 



The portion behind the Mlillerian evagination is formed from a pro- 

 longation of its distal end. It was found impossible to decide whether 

 the evagination was a single one, remaining after the degeneration of 

 others, or a compound one formed by fusion. That the Wolffian duct 

 contributes cells to the growing Mlillerian duct seems improbable from 

 the fact that, in the only region where the relationship between the two 

 ducts cannot be clearly distinguished, the Wolffian duct is very degener- 

 ate. Posterior to that point the two ducts seem to be wholly independent 

 of each other. 



Hyla versicolor. 



The phenomena exhibited in the degeneration of the pronephros of 

 Hyla differ from those in Rana in one important particular, — the fusion 

 of only the two posterior nephrostomes, instead of all three, to form a 

 " common nephrostome." In consequence of this peculiarity, the eva- 

 gination associated with the first nephrostome is allowed as free a devel- 

 opment as the anterior evagination in Amblystoma. The fusion of the 

 two posterior nephrostomes, however, brings about a crowding of tissue 

 similar to that in Rana, and as a consequence but one evagination 

 appears. This posterior evagination, from which the Mlillerian duct 

 develops, may represent the third (supposing one originally associated 

 with each nephrostome), or it may represent a fusion of the second and 

 third. Efforts to resolve it into two were unsuccessful in normal cases. 

 In the abnormal case in which the two posterior nephrostomes remained 

 separate, there was strong evidence of an evagination associated with 

 the second in addition to those unquestionable ones associated with the 

 first and third. 



In regard to the establishment of the adult ostium, Hyla resembles 

 Amblystoma far more closely than it does Rana. The cephalic displace- 

 ment of the opening of the young Mlillerian duct takes place along a 

 trough-like groove which extends from the posterior evagination to the 

 anterior, and is there continuous with a thickened band which extends 

 cephalad and ventrad toward the point where the adult ostium is situ- 

 ated. The process of displacement was observed only in its initial 

 stages, but at those stages it is accomplished by a progressive fusion of 

 the lips of the groove in a manner similar to that seen in Amblystoma, 

 and there can be little doubt that the remainder of the process is essen- 

 tially as in that form. 



