594 AMNIOTA. 



The mode of growth of the Miillerian duct in the posterior part of its 

 course will best be understood from the following description quoted from 

 the paper by Sedgwick and myself. 



" A few sections before its termination the Miillerian duct appears as a 

 well-defined oval duct lying in contact with the wall of the Wolffian duct 

 on the one hand and the geiminal epithelium on the other. Gradually, 

 however, as we pass backwards, the Miillerian duct dilates ; the external 

 ■wall of the Wolffian duct adjoining it becomes greatly thickened and 

 pushed in in its middle part, so as almost to touch the opposite wall of the 

 duct, and so form a bay in which the Miillerian duct lies. As soon as the 

 Miillerian duct has come to lie in this bay its walls lose their previous 

 distinctness of outline, and the cells comj)osing them assume a curious 

 vacuolated appearance. No well-defined line of separation can any longer 

 be traced between the walls of the Wolffian duct and those of the Miillerian, 

 but between the two is a narrow clear space traversed by an irregular net- 

 work of fibres, in some of the meshes of which nuclei are present. 



The Miillerian duct may be traced in this condition for a considerable 

 number of sections, the peculiar features above described becoming more 

 and more marked as its termination is approached. It continues to dilate 

 and attains a maximum size in the section or so before it disappears. A 

 lumen may be observed in it up to its very end, but is usually irregular in 

 outline and frequently traversed by strands of pi'otoplasm. The Miillerian 

 duct finally terminates quite suddenly, and in the section immediately 

 behind its termination the Wolffian duct assumes its noimal appearance, 

 and the part of its outer wall on the level of the Miillerian duct comes into 

 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 dvict (exclusive of the pronephi-os) appears to be 

 morphologically justified for the following reasons: (1) tliat 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 identi- 

 fied as such is in many respects similar to the pronephi'>os of the Am- 

 phibia. 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 ja-onephros 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 primitiv.e segmental duct, with the ])eritoneal 

 opening, which probably )-epresents the pronephros, is attached to the 

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

 verse is the ease. To explain the discontinuity of the pronephros with the 

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



