IN THE EMBRYO CHICK. 639 



duct in Elasmobranchii. We have already pointed out that in 

 Elasmobranchii the Miillerian duct is formed of two factors 

 (i) of the whole anterior extremity of the segmental duct, in- 

 cluding its abdominal opening ; (2) of a rod split off from the 

 ventral side of the segmental duct. In Birds the anterior part 

 (corresponding to factor No. i) of the Miillerian duct has a 

 different origin from the remainder ; so that if the development 

 of the posterior part of the duct (factor No. 2) were to proceed 

 in the same manner in Birds and Elasmobranchii, it ought to be 

 formed at the expense of the Wolffian (i.e. segmental) duct, 

 though in connection anteriorly with the head-kidney. And 

 this is what actually appears to take place. 



So far the homologies of the avian excretory system are 

 fairly clear; but there are still some points which have to be 

 dealt with in connection with the permanent opening of the 

 Miillerian duct, and the relatively posterior position of the head- 

 kidney. With reference to the first of these points the facts of 

 the case are the following : 



In Amphibia the permanent opening of the Mullerian duct 

 is formed as an independent opening after the atrophy of the 

 head-kidney. 



In Elasmobranchii the original opening of the segmental 

 duct forms the permanent opening of the Mullerian duct and no 

 head-kidney appears to be formed. 



In Birds the anterior of the three openings of the head-kidney 

 remains as the permanent opening of the Miillerian duct. 



With reference to the difficulties involved in there being 

 apparently three different modes in which the permanent opening 

 of the Mullerian duct is formed, we would suggest the following 

 considerations: 



The history of the development of the excretory system 

 teaches us that primitively the segmental duct must have served 

 as efferent duct both for the generative products and kidney 

 secretion (just as the Wolffian duct still does for the testicular 

 products and secretion of the Wolffian body in Elasmobranchii 

 and Amphibia) ; and further, that at first the generative products 

 entered the segmental duct from the abdominal cavity by one 

 or more of the abdominal openings of the kidney (almost cer- 

 tainly of the head-kidney). That the generative products did 



