504 DEVELOPMENT OF ELASMOBRANCH FISHES. 



sented on PL 21, fig. 9, or on a larger scale in fig. 9 A. In the 

 latter figure it is especially clear that while the wall on the 

 dorsal side of the lumen of each ureter is continuous with the 

 dorsal wall of the tubulus of its own segment, the wall on the 

 ventral side is continuous with the dorsal wall of the ureter of 

 the preceding segment. This feature in the ureters explains the 

 appearance of transverse sections in which the ureters are not 

 separate from each other, but form together a kind of ridge on 

 the dorsal side of the Wolffian duct, in which there are a series 

 of perforations representing the separate lumens of the ureters 

 (PI. 21, fig. 4). The peculiarities in the appearance of the 

 dorsal wall of the Wolffian duct in fig. 9 A, and the difference 

 between the cells composing it and those of the ventral wall, 

 become intelligible on comparing this figure with the repre- 

 sentation of transverse section in figs. 3 B and 3 C, and especially 

 in fig. 4. Most of the ureters continue to end blindly at the 

 close of stage N, and appear to have solid posterior terminations 

 like that of the Miillerian duct in Birds. 



By stage O all the ureters have become prolonged up to the 

 cloacal end of the Wolffian duct, so that the anterior one has a 

 length equal to that of the whole kidney proper. For the most 

 part they acquire independent openings into the end section of 

 the Wolffian duct, though some of them unite together before 

 reaching this. The general appearance of the hindermost of 

 them between stages N and O is shewn in longitudinal and 

 vertical section in PI. 21, fig. 8, u. 



They next commence to develope into complete and in- 

 dependent tubes by their side walls growing inwards and meet- 

 ing below so as to completely enclose their lumen. This is seen 

 already to have occurred in most of the posterior ureters in 

 PI. 21, fig. 8. 



Before stage P the ureters cease to be united into a con- 

 tinuous ridge, and each becomes separated from its neighbours 

 by a layer of indifferent tissue : by this stage, in fact, the ureters 

 have practically attained very nearly their adult condition. The 

 general features of a typical section through them are shewn on 

 PL 21, fig. 5. The figure represents the section of a female 

 embryo, not far from the cloaca. Below is the oviduct (od} t 

 Above this again is the Wolffian duct (w. d), and still dorsal to 



