184 The American Naturalist. [February, 



origin of the duct i- that it arise* from a solid proliferation of somato- 

 pleure . . . In so far as these authors maintain that the duct arises from 

 a solid profileration of mesoderm and acquires it- lumen secondarily I 

 entirely airree with them; hut my own observations on this point lead 

 me to conclude further that the duet arises throughout its entire 

 length from a continous 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. 



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

 ectodermal origin of tie due;, which i- to-dav advocated on so many 

 sides ... In my opinion the entire excretory system of the forms I 

 have studied unquestionably develops without any participation of the 

 ectoderm in its formation. The duct develops from mesoderm through- 

 out its entire length and at its posterior end, in Rana and Bufo at 



secondary and meaning 



The remaining part of ihe paper deal- with "those inferences of a 



"I conclude therefore that pronephros and mesonephros arc parts ot 

 one ancestral organ; that the glomeruli arc strictly homodynamous 

 with the glomus; that the entire tubular portion of the pronephros 

 is represented in the mesonephros ; that the cavity of a Malphigan 

 capsule and the nephrostomal canal connecting it with the body cavity 

 are detached portions of the eo loin, the equivalents of which are not 

 thus differentiated in the pronephros ; that the pronephros is developed 

 as a larval excretory organ; and that the period at which it appears 

 largely accouuts for its peculiarities of structure." 



The closing sections are devoted to a consideration of the evidence 

 which the development of the excretory system throws on the origin 

 of the vertebrates. On the whole, the evidence brought forward does 

 not add materially to the solution of the ancestry of the vertebra*!" 

 and such a theory can only be established by investigations wkie* 

 shall include in their scope the entire organization of the 1 D r U P 



