736 GENERAL CONCLUSIONS. 



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 (i) 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 tendency on the part of the anterior segmental 

 tubes to lose their excretory 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, (i) in the formation of a 

 completely distinct metanephros in both sexes, formed how r ever, 

 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 differentiation of parts. 



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

 factory proof of it has yet been offered. This proof has however, since 

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

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

 opposed to a phylogenetic development ; and the substitution of a direct for 



