hall: mesonephros and mullerian duct in amphibia. 95 



terior to the common nephrostome all traces of the peritoneal thickening 

 disappear. 



The further history of the various jDortions of this peritoneal thicken- 

 ing is as follows : the thickened disk dorsal to the first nephrostome 

 evaginates to form a pit, which generally is slightly anterior to the 

 nephrostome. Figures 87 to 93 (Plate 8) represent seven sections 

 through such an anterior evagination. Figures 87-92 represent succes- 

 sive sections; Figure 93 is three sections posterior to 92. 



This evagination is evidently comparable with the anterior evagination 

 of Amblystoma, and there are sometimes distinct signs of a proliferation 

 of cells from its distal end extending in a posterior direction, which may 

 be considered homologous with the "posterior cord of the anterior 

 evagination " of Amblystoma. In fact, it seems probable that these cell- 

 proliferations sometimes take on the form of more definite cords, for 

 there is occasionally seen in later stages a distinct cord — or even duct — 

 which begins back of the degenerating evagination and extends caudad, 

 close beneath the peritoneum. It is possible that such cords are really 

 • portions of the degenerating pronephric tubules, but I think not, as my 

 attention was called to them by the fact that, instead of being indefinite 

 tubules with large, pale cells, like those seen in the rest of the degener- 

 ating pronephros, they are clear-cut, with cells staining very darkly and 

 possessing little cytoplasm. 



Whether or not these cords form a part of the Mtillerian fundament, it 

 is clear that, as in Amblystoma, the anterior evagination and its cord play 

 no essential role in the formation of the Mullerian duct and are but relics 

 of a past history — for both evagination and cord disappear entirely. 



While the anterior evagination is disappearing, the thickeniug dorsal 

 to the common nephrostome grows more pronounced and in turn evagi- 

 nates. From its distal end cells are proliferated to form a cord, then a 

 duct, running straight caudad. This is of course the Mullerian duct. 

 Figure 84 (Plate 7) shows such a posterior evagination {evg. 2, 3), the 

 section being three sections posterior to the point where the second and 

 third nephrostomal tubules (shown in the figure) are joined to the peri- 

 toneum by a cord of cells representing the degenerating common nephro- 

 stome. Figure 78 (Plate 7) shows the posterior evagination at a later 

 stage. Sections posterior to the one figured show that it is continued 

 caudad as a duct which soon dwindles to a cord and then ceases. 



In Hyla versicolor the question as to whether the more anterior por- 

 tion of the Mullerian duct during its growth caudad takes cells from 

 the Wolffian duct, is easily answered in the negative, for the Wolffian 



