THE URETERS. 503 



and Wolffian duct takes place entirely at the expense of the 

 former. Owing to the length of this attachment, every trans- 

 verse section through the kidney proper at this stage either 

 presents a solid ridge of cells closely adhering to the dorsal side 

 of the Wolffian duct, or else passes through one of the openings 

 into the Wolffian duct. 



During stage M the original openings of the segmental tubes 

 into the Wolffian duct appear to me to become obliterated, and 

 at the same time the lumen of each ureter is prolonged into the 

 ridge of cells on the dorsal wall of the duct. 



Both of these changes are illustrated in my figures. The 

 fact of the obliteration of the original opening into the Wolffian 

 duct is shewn in longitudinal section in PI. 21, fig. 9, u, but 

 more conclusively in the series of transverse sections represented 

 on PI. 21, figs. 3 A, 3 B, 3 C. In the hindermost of these (3 C) 

 is seen the solid terminal point of a ureter, while the same 

 ureter possesses a lumen in the two previous sections, but ex- 

 hibits no signs of opening into the Wolffian duct. Sections 

 may however be met with which appear to shew that in some 

 instances the ureters still continue to open into the Wolffian 

 duct, but these I find to be rare and inconclusive, and am in- 

 clined to regard them as abnormalities. The prolongation of 

 the lumen of the ureters takes place in a somewhat peculiar 

 fashion. The lumen is not, as might be expected, completely 

 circumscribed by the wall of the ureter, but only dorsally and 

 to the sides. Ventrally it is closed in by the dorsal wall of the 

 Wolffian duct. In other words, each ureter is at first an in- 

 complete tube. This peculiarity is clearly shewn in the middle 

 figure of the series on PI. 21, fig. 3 B. 



During stages M and N the ureters elongate considerably, 

 and, since the foremost ones grow the most rapidly, they soon 

 come to overlap those behind. As each ureter grows in length 

 it remains an incomplete tube, and its lumen, though pro- 

 portionately prolonged, continues to present the same general 

 relations as at first. It is circumscribed by its proper walls 

 only dorsally and laterally ; its floor being formed in the case 

 of the front ureter by the Wolffian duct, and in the case of each 

 succeeding ureter by the dorsal wall of the ureter in front. 

 This is most easily seen in longitudinal sections, and is repre- 



