638 EXISTENCE OF A HEAD-KIDNEY 



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 de- 

 velopment from the segmental duct of Elasmobranchii is in 

 the absence 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 Mullerian duct in the Bird must be 

 considered 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 Mullerian duct of the Ichthy- 

 opsida. By Braun 1 , however, who from his researches on the 

 Lizard satisfied himself of the entire independence of the Mul- 

 lerian and Wolffian ducts in the Amniota, the Mullerian duct of 

 these forms is regarded as a completely new structure with no 

 genetic relations to the Mullerian duct of the Ichthyopsida. 

 Semper 2 , on the other hand, though he accepts the homology 

 of the Mullerian duct in the Ichthyopsida and Amniota, is of 

 opinion that the anterior part of the Mullerian duct in the 

 Amniota is really derived from the Wolffian duct, though he 

 apparently admits the independent growth of the posterior part 

 of the Mullerian duct. We have been led by our observations, 

 as well as by our theoretical deductions, to adopt a view exactly 

 the reverse of that of Professor Semper. We believe that the 

 anterior part of the Mullerian duct of Aves, which is at first the 

 head-kidney, and subsequently becomes the abdominal opening 

 of the duct, is developed from the peritoneal epithelium inde- 

 pendently of all other parts of the excretory system ; but that 

 the posterior part of the duct is more or less completely derived 

 from the walls of the Wolffian duct. This view is clearly in 

 accordance with our account of the facts of development in Aves, 

 and it fits in very well with the development of the Mullerian 



1 " Urogenital-System d. Reptilien," Arb. aus d. ^ool.-zoot. lust. Wurzburg, 



Vol. IV. 



2 Inc. cit. 



