500 DEVELOPMENT OF ELASMOBRANCH FISHES. 



I have satisfied myself, in conjunction with Mr Sedg\vick, that 

 this is not the case, and that the general account is in the main 

 true; but at the same time we have obtained -evidence which 

 tends to shew that the cells which form the Miillerian duct are 

 in part derived from the walls of the Wolffian duct. We propose 

 giving a full account of our observations on this point, so that I 

 refrain from mentioning further details here. It may however 

 be well to point out that, apart from observations on the actual 

 development of the Miillerian duct in the Bird, the fact of 

 its abdominal opening being situated some way behind the 

 front end of the Wolffian duct, is of itself a sufficient proof that 

 it cannot be the metamorphosed front extremity of the Wolffian 

 (= segmental) duct, in the same way that the abdominal opening 

 of the Miillerian duct is the front extremity of the segmental 

 duct in Elasmobranchs. 



Although the evidence I can produce in the case of the 

 Fowl of a direct participation of the Wolffian duct in the for- 

 mation of the Miillerian is not of an absolutely conclusive kind, 

 yet I am inclined to think that the complete independence of 

 the two ducts, if eventually established as a fact, would not of 

 itself be sufficient (as Semper is inclined to think) to disprove 

 the identity of the Miillerian duct in Birds and Elasmobranchs. 



We have, no doubt, almost no knowledge of the magnitude of 

 the changes which can take place in the mode of development of 

 the same organ in different types, yet this would have to be placed 

 at a very low figure indeed in order to exclude the possibility 

 of a change from the mode of development of the Miillerian 

 duct in Elasmobranchs to that in Birds. We have, it appears 

 to me, in the smallness of the portion of the segmental duct 

 which goes to form the Miillerian duct in Elasmobranchs, evidence 

 that a change has already appeared in this group in the direction 

 of a development of the Miillerian duct independent of the 

 segmental duct, and therefore of the \Volffian duct ; and it has 

 been in view of this consideration, that I have devoted so much 

 attention to the apparently unimportant point of how much 

 of the segmental duct was concerned in the formation of the 

 Miillerian duct. An analogous change, in a somewhat different 

 direction, would seem to be taking place in the development 

 of the rudimentary Miillerian duct in the male Elasmobranchs. 



