EXCRETORY ORGANS. 721 



duct finally terminates quite suddenly, and in the section immediately 

 behind its termination the Wolffian duct assumes its normal appearance, 

 and the part of its outer wall on the level of the Miillerian duct comesMnto 

 contact with the germinal epithelium." 



Before describing the development of the Miillerian duct in other 

 Amniotic types it will be well to say a few words as to the identifications 

 above adopted. The identification of the duct, usually called the Wolffian 

 duct, with the segmental duct (exclusive of the pronephros) appears to be 

 morphologically justified for the following reasons : (i) that it gives rise to 

 part of the Miillerian duct as well as to the duct of the Wolffian body ; 

 behaving in this respect precisely as does the segmental duct of Elasmo- 

 branchii and Amphibia. (2) That it serves as the duct for the Wolffian 

 body, before the Miillerian duct originates from it. (3) That it develops in a 

 manner strikingly similar to that of the segmental duct of various lower 

 forms. 



With reference to the pronephros it is obvious that the organ identified 

 as such is in many respects similar to the pronephros of the Amphibia. 

 Both consist of a somewhat convoluted longitudinal canal, with a certain 

 number of peritoneal openings ; 



The main difficulties in the homology are : 



(1) the fact that the pronephros in the Bird is not united with the 

 segmental duct ; 



(2) the fact that it is situated behind the front end of the Wolffian body. 

 It is to be remembered in connection with the first of these difficulties 



that in the formation of the Miillerian duct in Elasmobranchii the anterior 

 undivided extremity of the primitive segmental duct, with the peritoneal 

 opening, which probably represents the pronephros, is attached to the 

 Miillerian duct, and not to the Wolffian duct ; though in Amphibia the 

 reverse is the case. To explain the discontinuity of the pronephros with the 

 segmental duct it is only necessary to suppose that the segmental duct and 

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

 develop in the Bird as two independent structures a far from extravagant 

 supposition, considering that the pronephros in the Bird is undoubtedly 

 quite functionless. 



With reference to the posterior position of the pronephros it is only 

 necessary to remark that a change in position might easily take place after 

 the acquirement of an independent development, and that the shifting is 

 probably correlated with a shifting of the abdominal opening of the 

 Miillerian duct. 



The pronephros has only been observed in Birds, and is very 

 possibly not developed in other Amniota. The Mullerian duct 

 is also usually stated to develop as a groove of the peritoneal 

 epithelium, shewn in the Lizard in fig. 354, ind., which is con- 

 tinued backward as a primitively solid rod in the space between 

 B. in. 46 



