HALL: MESONEPHROS AND MULLERIAN DUCT IN AMriIIBL\.. 93 



duct in this region seems very improbable for the reason that by the 

 time the former reaches this region tlie degeneration of the Wolffian 

 duct has rendered it difficult of recognition if, indeed, it can be seen at 

 all. Posterior to the point where the fold containing the ducts reaches 

 its most nearly median position and turns again caudad, so that it is cut 

 transversely, the JMlillerian and Wolffian ducts are seen, in later stages, 

 imbedded in the lymphoid tissue at some little distance from each other. 

 The growth caudad of the Miillerian duct must be rather slow, for in a 

 young female frog which had completed its metamorphosis and left the 

 water, and in wliich the eggs were quite large and surrounded each by a 

 distinct follicular layer, the Miillerian duct had not reached the meso- 

 nephros. On the other hand, the oviducal welt, in which it lies when 

 developed, reaches as a well-defined ridge nearly to the cloaca, and is 

 generally widely sejmrated from the Wolffian duct. 



To return to the history of the anterior end of the duct; as in Am- 

 blystoma, the adult ostium is not formed directly from any of the evagi- 

 nations. Instead, the larval ostium is carried cephalad and ventrad, 

 utilizing the thickened band which, from its first appearance, takes that 

 course. The anterior extension of the Miillerian duct takes place at a 

 much earlier stage in Rana than in Amblystoma, but is accomplished by 

 fundamentally the same metliod, as follows : the thickened band which 

 extends from the evaginations cephalad and ventrad becomes broader, 

 and the dorsal edge folds downward as shown in Figure 79 (Plate 7).^ 

 The edge of the fold, which projects into the body cavity, becomes con- 

 tinuous caudad with tlie dorsal lip of an e vagi nation which is probably 

 the third, though it may be a composite of two or three evaginations. 

 Beginning at this point, the free edge of the fold then fuses with the 

 portion of the band which forms a part of the peritoneum, as shown in 

 Figure 80, which is two sections posterior to the one represented in 

 Figure 79. This bending down and fusion of the dorsal edge of the 

 band with its ventral margin which begins in the region of Figures 79 

 and 80, continues cephalad and ventrad until the opening reaches the 

 position of the adult ostium at the base of the lung. This process of 

 the forward and ventrad shifting of the ostium begins about the time of 

 the appearance of the fore legs. 



^ Anteriorly, where the band takes a ventral direction, this dorsal edge naturally 

 becomes anterior in position and hence folds over in a caudad direction. The sec- 

 tion figured cuts the band where it has turned ventrad, and hence quite obliquely. 

 The free edge of the fold, therefore, appears much broader tlian it would if it were 

 cut perpendicularly. 



VOL, XLV. — NO. 2 5 



