248 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



important work dealing with the morphology of the Vertebrate 

 kidney. It was in the opisthonephros of Elasmobranchs that 

 Sedgwick (1880) made his classical discovery — which forms the 

 foundation on which our present-day knowledge rests — that the 

 nephridial tube of the Vertebrate is a development of the coelomic 

 wall, of that part of it which we now call nephrotome or proto- 

 vertebral stalk. Since the date of Sedgwick's work the opistho- 

 nephros of Elasmobranchs has formed the subject of detailed studies 

 by Riickert, Rabl, van Wijhe, and other well-known investigators. 



Owing to the lower end of the myotome in these fishes becoming 

 displaced in a lateral direction, through the accumulation of mesen- 

 chyme between it and the mesial plane, the protovertebral stalk 

 becomes rotated outwards so as to assume a nearly horizontal position 

 (Fig. 134, A, nt), the originally dorsal end of the stalk becoming now 



Fig. 134. — Origin of opisthonephric tubule in Elasmobranchs. A, Pristiurus (after 

 C. Kabl, 1896) ; B, variation observed in Torpedo (after Riickert, 1888). 



A, dorsal aorta ; a.n.d, archinephric duct ; ect, ectoderm ; my, myotome ; nt, nephrotome ; 

 splc, splanchnocoele ; t, tubule rudiment. 



external, and the originally external side coming to be ventral. The 

 duct {a.n.d) thus comes to lie ventral to the nephrotomes instead 

 of being on their outer side as was the case originally. The nephro- 

 tomes become isolated from the myotomes by their ends next the 

 myotomes breaking up into mesenchyme. The result is that the 

 nephrotomes now form a series of blindly ending pocket-like pro- 

 jections of the coelomic epithelium which curve outwards dorsal to 

 the duct. 



Each pocket has an epithelial wall and it is noticeable that the 

 somatic portion of the wall is markedly thicker than the splanchnic, 

 the cells of the former being taller and more columnar in shape. As 

 development goes on it is found that the thicker more columnar 

 celled portion of the wall of the pocket extends for some distance on 

 to its dorsal wall, and this is interpreted by Riickert and Rabl as 

 meaning that the somatic epithelium is spreading inwards towards 

 the mesial plane, replacing splanchnic epithelium as it does so. In 

 view of what we know regarding the development of other groups it 

 seems more reasonable to explain the appearance as being expressive 



