iv MITLLEKIAN DUCT 243 



splanchnocoele ;it their anterior eud by an open funnel (ostium 

 t'ubae). There exists in some of the more archaic fishes what appears 

 to be distinct evidence that the Miillerian duct has been evolved 

 out of the tubulus juul duct of the pronephros and it will therefore 

 be convenient to consider this evidence now. 



The Elasmobranchs are the fishes in question. In Torpedo 

 (Riickert, 1888) as the pronephros degenerates its tubules become 

 reduced to the three hindermost. Of these* three the two posterior 

 degenerate while the other tubule E persists and its enlarged 

 nephrostonie becomes the coelomic funnel of the Miillerian duct. 

 Other workers (e.g. van Wijhe and Eabl), working on other Elasmo- 

 branchs (Pristiurus), trace back the coelomic funnel of the Miillerian 

 duct also to an opening derived from the pronephros and nephro- 

 stomal in its nature, but they believe the opening to be formed not 

 by the persistence of a single enlarged nephrostome but rather by 

 the fusion of three or four nephrostomes together. That it is morpho- 

 logically a single nephrostome is however rendered more probable by 

 what we now know regarding the development of the pronephros in 

 those of the more archaic fishes in which it develops as a functional 

 organ. It will be recalled, for example, how in Polypterus tubule E 

 (like B) becomes enlarged as compared with A, C, and D. A pro- 

 nephric tubule enlarged in this manner in correlation with purely 

 excretory needs would provide an obviously adequate beginning for 

 the evolution of a funnel for the transmission of the eggs like that 

 at the front end of the Miillerian duct. 



While the funnel of the Miillerian duct is nephrostomal in 

 origin the main part of the duct is developed in the Elasmobranchs 

 (Semper, 1875 ; Balfour, 1878) from the archinephric duct. The 

 latter undergoes a process of splitting from before backwards into a 

 dorsal and a ventral tube, the latter being at first a solid thickening 

 of the ventral wall of the archinephric duct. Of the two tubes so 

 formed the ventral is continuous with the pronephric funnel, while 

 the dorsal carries the openings of the kidney tubules farther back in 

 the series : the former becomes the Miillerian duct, the latter persists 

 as the functional duct of the opisthonephros (Fig. 133, C, W.d). 



This mode of development is satisfactorily explained by the 

 assumption that the relatively archaic fishes in which it occurs are 

 repeating the process by which the Miillerian duct arose in evolu- 

 tion. Such a splitting of an originally common duct into two, so 

 as to separate the routes by which two different products reach the 

 exterior, is probably of frequent occurrence in evolution. Good 

 examples are seen in the splitting of the common genital duct of 

 hermaphrodite gasteropods (e.g. the ordinary snails) to form a 

 separate oviduct and vas deferens. It appears then justifiable to 

 accept as a working hypothesis that the Miillerian duct arose in 

 evolution by being split oft' from the archinephric duct and that its 

 coelomic funnel is a persistent pronephric funnel. 



Turning to Vertebrates other than Elasmobranchs, well-marked 



