606 GENERAL CONCLUSIONS. 



after falling into the body cavity, might be taken up by tlie open extremi- 

 ties of some of the peritoneal funnels, and how such open funnels might 

 have groove- like prolongations along the mesorchium, which might even- 

 tually be converted into ducts. Ontogeny does not however altogether 

 favour this view of the origin of the testicular network. It seems to me 

 nevertheless the most probable view which has yet been put forward. 



The mode of transportation of the semen by means of the mesonephric 

 tubules is so peculiar as to render it highly improbable that it was twice 

 acquired, it becomes therefore necessary to suppose that the Amphibia and 

 Amniota inherited this mode of transportation of the semen from the same 

 ancestors as the Elasmobranchii. It is remarkable therefore that in the 

 Ganoidei and Dipnoi this arrangement is not found. 



Either (1) the arrangement (found in the Ganoidei and Dipnoi) of the 

 Miillerian duct serving for both sexes is the primitive arrangement, and 

 the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary 

 condition, which has originated at a stage in the evolution of the Vertebrata 

 when some of the segmental tubes had begun to serve as the efferent ducts 

 of the testis, and has resulted in consequence of a degeneration of the latter 

 structures. Although the second alternative is the more easy to reconcile 

 with the affinities of the Ganoid and Elasmobranch types, as indicated by 

 the other features of their organization, I am still inclined to accept the 

 former ; and consider that the incomplete splitting of the segmental duct in 

 Ganoidei is a strong argument in favour of this view. 



Metanephros. With the employment of the Wolffian duct to 

 transport the semen there seems to be correlated (I) a tendency of 

 the posterior segmental tubes to have a duct of their own, in which 

 the seminal and urinary fluids cannot become mixed, and (2) a ten- 

 dency on the part of the anterior segmental tubes to lose their excre- 

 tory function. The posterior segmental tubes, when connected in 

 this way with a more or less specialised duct, have been regarded in 

 the preceding pages as constituting a metanephros. 



This differentiation is hardly marked in the Anura, but is well 

 developed in the Urodela and in the Elasmobranchii ; and in the 

 latter group has become inherited by both sexes. In the Amniota it 

 culminates, according to the view independently arrived at by 

 Semper and myself, (1) in the formation of a completely distinct 

 metanepliros in both sexes, formed however, as shewn by Sedgwick, 

 from the same blastema as the Wolffian body, and (2) in the atrophy 

 in the adult of the whole Wolffian body, except the part uniting the 

 testis and the Wolffian duct. 



The homology between the posterior metanephridian section of the 

 Wolffian body, in Elasmobranchii and Urodela, and the kidney of the 

 Amniota, is only in my opinion a general one, i.e. in both cases a common 

 cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more 

 or less similar difiterentiation of parts. 



Fiirbringer has urged against Semper's and my view that no satis- 

 factory proof of it has yet been offijred. This j)roof has however, since 

 Furbringer wrote his paper, been supplied by Sedgwick's observations. 

 The development of the kidney in the Amniota is no doubt a direct as 



