158 DEVELOPMENT OF THE FROG'S EGG [Ch. XIV 



fusion extend in vertical lines from the nephrostomal margin 

 of the pouch nearly to its ventral border, where there is feft 

 an unfused and therefore continuous longitudinal tract con- 

 stituting the canal which I have called the collecting trunk." i 

 Field continues, "In opposition to this view, I would maintain : 

 (1) That the first trace of the excretory system consists of a 

 solid proliferation of somatopleure, the pronephric thicken- 

 ing; (2) that the lumen of the system arises secondarily; and 

 (3) that the pronephric tubules do not appear in consequence of 

 the local fusion of the walls of a widely open pouch, but that 

 they are differentiated at an early stage from the hitherto 

 indifferent pronephric thickening." 



The pronephric duct of the Amphibia arises, according to 

 one view, as we have seen above, from an evagination of soma- 

 topleure, its lumen being therefore a detached portion of the 

 body-cavity. A second view of the origin of the duct is, that 

 It arises from a solid proliferation of somatoj^leure. Field 

 agrees with the latter view. A third view maintains that the 

 duct is ectodermic in origin. Field has shown, however, that 

 in the Amphibia the excretory system develops most probably 

 without any participation of the ectoderm in its formation. 



"This view of the development of the pronephros, although suggested by 

 Wilh. Miiller, was first described in detail by Goette for Bombinator, and was 

 later extended to other Amphibia by the researches of Furbringer. It has been 

 entirely confirmed by Wichmann, by Hoffmann, and still more recently by 

 Marshall and Bles." (Field, '91, page 281.) 



