WOLFFIAN DUCT AND BODY IN THE CHICK. 37 



body are capable of shifting their position according to the 

 wants of the particular species. 



We know very well other organs can do this, and I need only 

 mention the anus placed so near the head in frogs, and so far 

 off in Ccecilia, and it seems only probable that an important 

 gland like the kidney should be capable of acquiring a position 

 and arrangement of its constituent parts different from the posi- 

 tion of their development, if it is advantageous for the per- 

 formance of the function of the organ. 



The evidence which at the first look appeared so strong 

 against the primitiveness of the Elasmobranch arrangement of 

 one primary tubule to each segment proves on examination to 

 lose a great part of its force. 



I now come to a difficulty which apparently at present presents 

 an insuperable obstacle to a successful solution of the question 

 under consideration, viz. What was the structure and deve- 

 lopment of the excretory system of the ancestral Vertebrate ? 



Assuming that the development of the Elasmobranch mesone- 

 phros presents primitive features in the two details already con- 

 sidered, its development in a third particular can by no means 

 be assumed to be primitive. The fact that the segmental 

 duct develops independently of the tubules cannot, in the 

 present state of our knowledge, be regarded as primitive. 

 Objections of precisely the same kind as those used in arguing 

 against the development of the tubules in Amphibia, &c., being 

 primitive present themselves here. 



Any phlyogenetic hypothesis which presents difficulties from 

 a physiological standpoint must be regarded as very provisional 

 indeed. The physiological difficulty present in the conception 

 that in the evolution the mesonephros has arisen by the fusion of 

 two distinct parts, viz. the duct and tubule, is so great that 

 until facts are brought forward to show a different origin we 

 must consent to admit our total ignorance on this point. I 

 think that the observations recorded in the first part of this 

 paper on the development of the Avian Wolffian duct and 

 anterior tubules are of great interest in this relation. Here 

 we have the Wolffian duct and tubules developing in continuity 

 in the anterior part of the excretory system, which has been 

 always admitted to present the most primitive development. 

 But this point I must again keep for later consideration. 



So far, then, the following conclusions have been reached — the 

 development of the mesonephros of Elasmobranchii is in part 

 primitive (tubules), and in part very much modified, while the 

 development of the mesonephros of Amphibia, Teleostei, &c., 

 is in all respects modified. 



Turning to the development of the segmental duct, we find 



