IN THE EMBRYO CHICK. 637 



a small lumen is present in its middle part In the succeeding 

 stages the lumen of the duct gradually extends backwards 

 and forwards, the duct itself also passes inwards till it acquires 

 its final position close to the peritoneal epithelium ; at the same 

 time its hind end elongates till it comes into connection with 

 the cloacal section of the hind-gut. It should be noted that the 

 duct in its backward growth does not appear to come into con- 

 tinuity with the subjacent mesoblast, but behaves in this respect 

 exactly as does the segmental duct in Elasmobranchii (vide note 

 on p. 634). 



The question which we propose to ourselves is the follow- 

 ing : What are the homologies of the parts of the Avian urino- 

 genital system above enumerated ? The Wolffian duct appears 

 to us morphologically to correspond in part to the segmental 

 duct 1 , or what Furbringer would call the duct of the head-kidney. 

 This may seem a paradox, since in birds it never comes into 

 relation with the head-kidney. Nevertheless we consider that 

 this homology is morphologically established, for the following 

 reasons : 



(1) That the Wolffian duct gives rise (vide supra, p. 631) to 

 the Mullerian duct as well as to the duct of the Wolffian body. 

 In this respect it behaves precisely as does the segmental duct 

 of Elasmobranchii and Amphibia. That it serves as the duct for 

 the Wolffian body, before the Mullerian duct originates from it, 

 is also in accordance with what takes place in other types. 



(2) That it develops in a strikingly similar manner to the 

 segmental duct of Elasmobranchii. 



We stated expressly that the Wolffian duct corresponded 

 only in part to the segmental duct. It does not, in fact, in our 

 opinion, correspond to the whole segmental duct, but to the 

 segmental duct minus the anterior abdominal opening in Elas- 

 mobranchii, which becomes the head-kidney in other types. In 

 fact, we suppose that the segmental duct and head-kidney, which 



1 The views here expressed about the Wolffian duct are nearly though not exactly 

 those which one of us previously put forward (" Urinogenital Organs of Vertebrates," 

 &c., pp. 45 46) [This edition, pp. 164, 165], and with which Fiirb ringer appears exactly 

 to agree. Possibly Dr Furbringer would alter his view on this point were he to accept 

 the facts we believe ourselves to have discovered. Semper's view also differs from 

 ours, in that he believes the Wolffian duct to correspond in its entirety with the 

 segmental duct. 



