hall: mesoxephros and mullepjan duct in amphibia. 115 



Rana sylvatica. 



Preceding the degeneration of the pronephros, the three nephrostomes 

 move closer and closer together and finally open into a common out- 

 pocketing of the coeloin, which forms a narrow, ciliated canal closely- 

 resembling the primary nephrostomes. This "common nephrostome " 

 is comparable to the " nephrostomal cavity " in Amblystoma in that, by 

 its closure, it severs the connection of the nephrostomes with the 

 coelom, just as the closure of the "nephrostomal cavity" shuts off the 

 first nephrostome. 



As a result of the migration of the nephrostomes to a common point, 

 the tissue of the peritoneum which was immediately ventral to each is 

 represented by tissue ventral to the common nephrostome. It is suo-- 

 gested that this tissue contains the fundaments of the three Miillerian 

 evaginations which should theoretically be present if there is a correla- 

 tion between the number of evaginations and nephrostomes, as the con- 

 dition in Amblystoma suggests. As a consequence of the condensation 

 of the tissue of the three fundaments, the latter are generally repre- 

 sented by an irregular mass of proliferated cells. This mass is, however, 

 elongated in the direction of the long axis of the body and usually 

 shows two or three distinct regions of heightened proliferation, or 

 may even show more than one distinct evagination. A fundament is 

 figured in which tliere are two distinct evaginations and a proliferated 

 cone which may represent a third. An additional proof that there was 

 originally a MiiUerian evagination associated with each of the three 

 nephrostomes is seen in the fact that in one larva, in which the fusion 

 of the nephrostomes had been retarded, there is one distinct evagination 

 beneath the common opening of the first and second nephrostomes and 

 a thickened disk, probably representing another, beneath the third 

 nephrostome, which opens at some distance behind the other two. 



In the case just mentioned, there are two thickened bands which 

 fuse and extend cephalad and ventrad. In normal cases there is a single 

 band. As the shelf in Rana sylvatica is a very transitory structure of 

 small extent, the curving of the thickened band in passing forward to the 

 sub-glomerular cavity, which is such a prominent feature in Amblystoma, 

 is barely represented. Soon after the appearance of the evaginations, 

 the band forms a conspicuous groove or fold, open below, so that cross- 

 sections of it have the form of an inverted V. The dorso-median wall 

 of the groove later becomes continuous with the dorso-median lip of the 

 single remaining opening of the Mlillerian duct, and the latter is carried 



