NEPHRIDIAL ORGANS 



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new generations of tubules, associated with and opening into them, until 

 they form the bulky kidney of the adult. 



M.d. 



Wd. 



-U. 



FIG. 128. 



Diagram to illustrate the evolution of the nephridial organs of vertebrates. A, Supposed 

 primitive condition. B, A group of tubules in front have become differentiated as the pronephros 

 from the remainder which constitute the opisthonephros, while the portions of splanchnocoele 

 immediately adjacent to the nephros tomes have become partially or completely separated off 

 from the main splanchnocoele as the cavities of the Malpighian bodies. C, Certain of the Malpighian 

 bodies are connected with the testis, their tubules serving to transmit the male gametes ; the 

 pronephros is reduced to a single greatly widened tubule which forms the internal funnel of the 

 oviduct ; the archinephric duct is splitting into the Wolffian duct continuous with the opistho- 

 nephric tubules and the Miillerian duct (oviduct) continuous with the persisting pronephric tubule. 

 D, The hindmost tubule has sprouted to form a large tree-like mass of tubules the metanephros, 

 while its backward continuation is splitting off as the ureter from the Wolffian duct or vas deferens. 

 The opisthonephric tubules connected with the testis form the epididymis. a.n.d, Archinephric 

 duct ; M, Malpighian body ; M.d, Mullerian duct ; mn, metanephros ; op, opisthonephros ; os, in- 

 ternal funnel of oviduct ; p.c, peritoneal canal ; p.f, peritoneal funnel ; pn, pronephros ; T, testis ; 

 U, ureter ; v.e, vas efferens ; W.d, Wolffian duct. 



