274 URETERS. 



sections, and is represented on PI. XX. 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 trans- 

 verse 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 per- 

 forations representing the separate lumens of the ureters 

 (PL XX. 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 fig. 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 longi- 

 tudinal and vertical section in PI. XX. fig. 8, u. 



They next commence to develope into complete and inde- 

 pendent tubes by their side walls growing inwards and meeting 

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

 already to have occurred in most of the posterior ureters in 

 PI. XX. fig. 8. 



Before staofe 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 XX. fig. 5. The figure represents the section 

 of a female embryo, not far from the cloaca. Below is the 



