DEVELOPMENT OF ELASMOBBANCH FISHES. 278 



segmental tubes and Wolffian duct takes place entirely at the 

 expense of the former. Owing to the length of this attach- 

 ment, every transverse 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 ob- 

 literated, 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. XX, fig. 9, w, but 

 more conclusively in the series of transverse sections represented 

 on PI. XX. fig. 3 A, 3 B, 3 0. In the hindermost of these 

 (3 0) is seen the solid terminal point of a ureter, while the 

 same ureter possesses a lumen in the two previous sections, but 

 exhibits 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 

 inclined 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. Yentraily it is closed in by the dorsal wall of 

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

 incomplete tube. This peculiarity is clearly shewn in the 

 middle figure of the series on PI. XX. 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 

 proportionately 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 



