THE OKGASS OF THE MIDDLE GEEM-LAYER. 



365 



similar structures of the excretory organs of segmented Worms. In 

 the most of the Vertebrates, however, special nephridial funnels are 

 no longer developed, inasmuch as the mesonephric tubules soon after 

 their origin completely detach themselves from the epithelium of the 

 body-cavity as well as from the primitive 

 segments, and thereby lose all relation to 

 the body cavity. 



A mesonephros in the simple form in 

 which it is at first produced develop- 

 mentally is retained permanently only 

 in Bdellostoma, a representative of the 

 Cyclostomes. It here consists, as Jo- 

 hannes MtJLLEB has shown, of an elon- 

 g£ited canal (fig. 206 A and B a) and 

 short transverse tubules (6), which open 

 into it at short intervals. The latter are 

 no longer connected with the body-cavity 

 by means of a nephridial funnel, but they 

 enclose a vascular glomerulus at their 

 blind end (fig. 206 JB c), which is some- 

 what set off by a constriction. 



In all remaining Vertebrates the meso- 

 nephros is metamorphosed into a more 

 voluminous and more complicated organ. 

 For the originally short tubules, which 

 run transversely into the mesonephric 

 duct, begin to grow in length, and at the 

 same time to be thrown into numerous 

 folds (fig. 207 s.t). Moreover there are 

 formed mesonephric tubules of a second 

 and third order. These again are also 

 formed independently of the mesonephric 

 duct dorsal to the first-formed transverse 

 tubules ; their blind ends approach the 

 primary urinary tubule and join its ter- 

 minal part, which is thereby converted 



into a collecting tube. At the same time a Malpighian body is 

 formed on each of them also. 



Still more exhaustive investigations concerning the formation of the second 

 arj' and tertiary mesonephric tubules, especially for the higher Vertebrates, 

 appear to me to be desiiable. In the Selachians, according to the statements 



Fig. 206.— Farts of the mesone- 

 phros of Myxine, after J. 



MiJLLER. 



ui, Mesonephric duct ; b, mesone- 

 phric tubules ; c, glomerulus ; 

 d, afferent artery ; e, efferent 

 artery. 



B a part of A more highly mag- 

 ni^ed. 



