18 F. M. BALFOUR. 



the Miillerian duct of the Iclithyopsida. Semper^ on the other 

 hand, though he accepts the homology of the Miillerian duct 

 in the Ichthyopsida and Amniotaj is of opinion that the anterior 

 part of the Miillerian duct in the Amniota is really derived from 

 the Wolffian duct, though he apparently admits the independent 

 growth of the posterior part of the Miillerian duct. We have 

 been led by our observations, as well as by our theoretical de- 

 ductions, to adopt a view exactly the reverse of that of Professor 

 Semper. We believe that the anterior part of the Miillerian 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 independently 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 V\^olffian 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 develop- 

 ment of the Miillerian duct in Elasmobranchii. We have already 

 pointed out that in Elasmobranchii the Miillerian duct is formed 

 of two factors — (1) of the whole anterior extremity of the seg- 

 mental duct, including 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. 1) of the Miillerian 

 duct has a diff'erent 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 Elasmo- 

 branchii, 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 placo. 



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 Miillerian 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 Miillerian 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 



' Loc. cit. 



