502 DEVELOPMENT OF ELASMOBRANCH FISHES. 



anterior segments as a distinct gland, which was spoken of in 

 the description of the adult as the kidney proper, while the 

 unaltered preceding segments of the kidney were spoken of as 

 the Wolffian body. 



It will be remembered that each segment of the embryonic 

 kidney consists of four divisions, the last or fourth of which 

 opens into the Wolffian duct. The changes which take place 

 in the hindermost. ten or eleven segments, and cause them to 

 become distinguished as the kidney proper, concern alone the 

 fourth division of each segment, which becomes prolonged back- 

 wards, and its opening into the Wolffian duct proportionately 

 shifted. These changes affect the foremost segments of the 

 kidney much more than the hindermost, so that the fourth 

 division in the foremost segments becomes very much longer 

 than in the hindermost, and at last all the prolongations of the 

 kidney segments come to open nearly on the same level, close 

 to the cloacal termination of the Wolffian duct (PL 21, fig. 8). 

 The prolongations of the fourth division of the kidney-segments 

 have already (p. 481) been spoken of in the description of the 

 adult as ureters, and this name will be employed for them in the 

 present section. 



The exact manner in which the changes, that have been 

 briefly related, take place is rather curious, and very difficult 

 to unravel without the aid of longitudinal sections. First of all, 

 the junction between each segment of the kidney and the 

 Wolffian duct becomes so elongated as to occupy the whole 

 interval between the junctions of the two neighbouring seg- 

 ments. The original opening of each tube into the Wolffian 

 duct is situated at the anterior end of this elongated attach- 

 ment, the remaining part of the attachment being formed solely 

 of a ridge of cells on the dorsal side of the Wolffian duct. The 

 general character of this growth will be understood by com- 

 paring figs. 7 a and 7 b, PL 21 two longitudinal vertical sec- 

 tions through part of the kidneys. Fig. 7 a shews the normal 

 junction of a segmental tube with the Wolffian duct in the 

 Wolffian body, while in figure 7 b (r. 11) is shewn the modified 

 junction in the region of the kidney proper in the same embryo. 

 The latter of these figures (fig. 7 b) appears to me to prove that 

 the elongation of the attachments between the segmental tubes 



