104 bulletin: museum of comparative zoology. 



coelomic epithelium. This retrogressive process is continued until the 

 ventral, caudally directed portion of the duct has been obliterated, and 

 the opening — now the ostium abdominale — occupies its adult position. 



In the degeneration of the pronephros, the anterior nephrostome is 

 the first to disappear, the second next, and, after some time, the third. 

 It is in the vicinity of the surviving third nephrostome that the fusion 

 of the pronephric duct with the peritoneum takes place. 



I think there is no doubt that Hoftmann mistook the developing 

 MllUerian duct for the anterior end of the pronephric duct. His 

 description of the mode of formation of the duct anterior to the point 

 where it first opens into the body cavity is in accord with what I found 

 in Rana sylvatica, with the exception of the obliteration of a temporary, 

 cephalic portion. The description of the degeneration of the pronephros 

 is, of course, entirely different from my own results. 



According to MacBridc ('91, '92) there is no trace of the Miillerian duct 

 in " tlie frog " until the tadpole has lost all its larval organs except the 

 tail. The first trace of the duct is ventral to the only remaining nepliro- 

 stome which, from its position, is probably the first. The fundament 

 consists of a groove lined with columnar cells and open below. " This 

 columnar epithelium is continued out over the surface of the pronephros 

 and beyond it, as described by Hoffmann." The groove changes, pos- 

 teriorly, into a canal which ends in a thickening of the peritoneum. 

 Anteriorly, in later stages, the groove extends ventrad and becomes 

 closed to form a canal " opening somewhat ventrally." Posteriorly, the 

 thickening of the peritoneum, which constitutes the duct, runs back 

 along a line of columnar epithelium which extends along the outer 

 border of the mesonephros. MacBride calls the cord of cells which 

 forms the duct a "thickening of the peritoneum" because it appears to 

 be derived from the peritoneum, although he cannot be certain of the 

 origin of its cells on account of the lymphoid tissue with which the outer 

 boundary of the mesonephros is filled. In describing the cord he says 

 " it appears [in cross-sections] as a nodule of deeply staining tissue, the 

 outermost cells of Avhich pass at the side into the ordinary epithelium." 

 The duct is formed from this cord by the rearrangement of some of its 

 cells in a stellate manner. Anterior to the mesonephros the cord " grows 

 back with some regularity," but it appears posterior to the mesonephros 

 long before it does along the side of that organ. ^ The lumen appears 

 first anterior, then posterior, to the mesoneplu'os. Posteriorly it appears 



1 One cannot help suspecting that MacBride confused the Miillerian duct and 

 oviducal welt. 



