EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 17 



The question which we propose to ourselves is the following : — 

 What are the homologies of the parts of the Avian urinogenital 

 system above enumerated ? The Wolffian duct appears to us mor- 

 phologically to correspond in part to the segmental duct/ or 

 what Fiirbringer 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. 12) to the 

 Miillerian 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 Miillerian 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 duot minus the anterior abdominal opening in 

 Elasmobranchii, which becomes the head-kidney in other types. 

 In fact, we suppose that the segmental duct and head- 

 kidney, which in the Ichthyopsida develop as a single formation, 

 develop in the Bird as two distinct structures — one of these 

 known as the Wolffian duct, and the other the head-kidney. 

 If our view about the head-kidney is accepted the above position 

 will hardly require to be disputed, but we may point out that the 

 only feature in which the Wolffian duct of the Bird differs in deve- 

 lopment from the segmental duct of Elasmobranchii is in the ab- 

 sence of the knob, which forms the commencement of the segmental 

 duct, and in which the abdominal opening is formed ; so that 

 the comparison of the development of the duct in the two types 

 confirms the view arrived at from other considerations. 



The head-kidney and Miillerian duct in the Bird must be con- 

 sidered together. The parts which they eventually give rise to 

 after the atrophy of the head-kidney have almost universally been 

 regarded as equivalent to the Miillerian duct of the Ichthyopsida. 

 By Braun,^ however, who from his researches on the lizard satisfied 



* The views here expressed about the Wolffian duct are nearly thouj^h not 

 exactly those which one of us previously put forward (' Urinogenital Organs 

 of Vertebrates,' &c., p. 45-16), and with which Eiirbringer appears exactly 

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

 to accept the facts we believe ourselves to have discovered. Scraper's view 

 also differs from ours, in that he believes the Wolffian duct to correspond 

 in its entirety with the segmental duct. 



- "Urogenital System d. Reptilien." ' Arb. aus d. zool.-zoot. Inst.' 

 Wiirzburg, vol. iv, 



2 



