490 DEVELOPMENT OF ELASMOBRANCH FTSHES. 



Development of tfie Segmental Tubes. 



On p. 345, et seq. an account was given of the first formation 

 of the segmental tubes and the segmental duct, and the history 

 of these bodies was carried on till nearly the period at which it 

 is taken up in the exhaustive Memoir of Professor Semper. 

 Though the succeeding narration traverses to a great extent the 

 same ground as Semper's Memoir, yet many points are treated 

 somewhat differently, and others are dealt with which do not 

 find a place in the latter. In the majority of instances, attention 

 is called to points on which my results either agree with, or are 

 opposed to, those of Professor Semper. 



From previous statements it has been rendered clear that at 

 first the excretory organs of Elasmobranchs exhibit no division . 

 into Wolffian body or kidney proper. Since this distinction 

 is merely a question of the ducts, and does not concern the 

 glandular tubuli, no allusion is made to its appearance in the 

 present section, which deals only with the glandular part of the 

 kidneys and not with their ducts. 



Up to the close of stage K the urinogenital organs consist 

 of a segmental duct opening in front into the body-cavity, and 

 terminating blindly behind in close contact with the cloaca, and 

 of a series of segmental tubes, each opening into the body-cavity 

 on the inner side of the segmental duct, but ending blindly at 

 their opposite extremities. It is with these latter that we have 

 at present to deal. They are from the first directed obliquely 

 backwards, and coil close round the inner and dorsal sides of the 

 segmental duct. Where they are in contact (close to their open- 

 ings into the body-cavity) with the segmental duct, the lumen of 

 the latter diminishes and so comes to exhibit regular alternations 

 of size. This is shewn in PI. 12, fig. 18 s. d. At the points where 

 the segmental duct has a larger lumen, it eventually unites with 

 the segmental tubes. 



The segmental tubes rapidly undergo a series of changes, the 

 character of which may be investigated, either by piecing together 

 transverse sections, or more easily from longitudinal and vertical 

 sections. They acquire a A -shaped form with an anterior limb 

 opening into the body-cavity and posterior limb, resting on a 



