THE URINOGENITAL ORGANS OF VERTEBRATES. 165 



The second mode of stating the difference in development in 

 the two cases represents the embryological facts of the bird far 

 better than the other method. 



It explains why the Wolffian duct appears earlier than the 

 Miillerian and not at the same time, as one might expect ac- 

 cording to the other way of stating the case. If the Wolffian 

 duct is equivalent to the segmental duct of Selachians, it must 

 necessarily be the first duct to develope ; and not impro- 

 bably the development of the Miillerian duct would in birds 

 be expected to occur at the time corresponding to that at 

 which the primitive duct in Selachians became split into two 

 ducts. 



It probably also explains the similarity in the mode of de- 

 velopment of the Wolffian duct in birds and the primitive duct 

 of the kidneys in Selachians. 



This way of stating the case is also in accordance with 

 theoretical conclusions. As the egg-bearing function of the 

 Miillerian duct became more and more confirmed we might ex- 

 pect that the adult condition would impress itself more and 

 more upon the embryonic development, till finally the Miil- 

 lerian duct ceased to be at any period connected with the 

 kidneys, and the history of its origin ceased to be traceable in 

 its development. This seems to have actually occurred in the 

 higher vertebrates, so that the only persisting connection be- 

 tween the Miillerian duct and the urinary system is the brief but 

 important junction of the two at their lower ends on the sixth 

 or seventh day. This junction justly surprised Waldeyer (Eier- 

 stock it. Ei, p. 129), but receives a complete and satisfactory 

 explanation on the hypothesis given above. 



The original development of the segmental tubes is in the 

 bird solely retained in the tubules of the Wolffian body arising 

 independently of the Wolffian duct, and I have hitherto failed 

 to find that there is a distinct division of the Wolffian bodies 

 into segments corresponding with the vertebral segments. 



I have compared the permanent kidneys to the lower por- 

 tion of the kidneys of Selachians. The identity of the ana- 

 tomical condition of the adult Selachian and embryonic bird 

 which has been already pointed out speaks strongly in favour 

 of this view ; and when we further consider that the duct of 



