290 BULLETIN OF THE 



view, however, affirms that the posterior end of the duct grows back- 

 ward free from adjacent tissue, the cellular material being wholly de- 

 rived from an anterior proliferation. For Selachii this method of origin 

 has been maintained by Balfour ('78), and for Amniotes by a large num- 

 ber of observers ; e. g. Weldon ('83) and Mihalkovics ('85) in Reptiles ; 

 Gasser ('77), Sedgwick ('81), Schmiegelow('81 and '82), and Janosik ('85), 

 in Birds ; Benson ('83) and Martin ('88), in Mammals. Gasser ('82) 

 believes that the segmental duct in Alytes has no direct connection with 

 the mesoderm, posterior to the pronephros; but he was unable to ex- 

 clude with certainty the possibility that the somatopleure immediately 

 behind the pronephros might take some part in the formation of the 

 duct. Mollier ('90, p. 22G) moreover asserts that such a participation 

 actually takes place in the two somites following those in which the pro- 

 nephros is formed, but that the posterior portion of the duct probably 

 grows back from this point independently of the mesoderm. 



In so far as these authors maintain that the duct arises from a solid 

 proliferation of mesoderm and acquires its lumen secondarily, I entirely 

 agree with them ; but my observations on this point lead me to conclude 

 further that the duct arises throughout its entire length from a continu- 

 ous thickening of somatopleure, and that the only free growth which 

 occurs in the Amphibia studied by me is for the purpose of effecting a 

 union with the cloaca. In assuming this position, I am aware of being 

 in conflict with prior observations on Amphibia, and with the more recent 

 accounts of the development in other groups ; it seems to me, however, 

 that satisfactory evidence in favor of this mode of origin has been ad- 

 duced in the descriptive part of this paper. 



Finally it remains for me to consider the third view, that of the ecto- 

 dermal origin of the duct, which is to-day advocated on so many sides. 

 As early as 1855 Bemak expressed himself as dissatisfied with the deriva- 

 tion of the excretory system from the mesoderm, although this mode of 

 origin was confirmed by his own observations. A decennium later His 

 C65 h , pp. 160-1G2) maintained that, in the Chick, the Wolffian and 

 Miilleriau ducts both arise as folds of the ectoderm ; but he abandoned 

 this position later ('08, p. 119), when it had been shown by Boruhaupt 

 ('67) and Dursy ('G7) to be untenable. He then endeavored to interpret 

 the facts in harmony with his theoretical conceptions by maintaining that 

 the cells from which the Wolffian and Mullerian ducts arose were pri- 

 marily derived from the ectoderm, a view which was likewise adopted by 

 Waldeyer ('70). Meantime Hensen ('66) had indorsed the view of a 

 direct origin from the ectoderm. He states ('66, p. 81, foot-note) that 



