Xo. 5. — The Development of the Pronephros and Segmental Duct 

 in Amphibia. By Herbert H. Field. 1 



I. Introduction 



II. Descriptive Part 203 



A. Rana 204 



Stage 1 204 



Stage II 209 



Stage III 213 



Stage IV 219 



Stage V 227 



Stage VI 237 



Stage VII 241 



B. Bufo 242 



Stage 1 242 



Stage II 243 



Stage III 244 



Stage IV 245 



Stage V 246 



Contents. 



Page 

 201 



Page 



C. Amblystoma 247 



Stage 1 248 



Stage II 250 



Stage III 250 



Stage IV 252 



Stage V 252 



Stage VI 257 



III. General Discussion .... 262 



The Kidneys of Amphioxus 262 

 The Pronephros of the Cra- 



niota . 266 



The Segmental Duct ... 288 



Organogenetic Conclusions . 295 



Phylogenetic Conclusions . 307 



IV. Bibliography 323 



V. Explanation of Figures . . . 341 



I. Introduction. 



The studies upon which this paper is based were undertaken with the 

 purpose of determining the relation which the urogenital system bears 

 to the germinal layers in Amphibia. At the time when they were begun, 

 especial interest in this topic had been awakened by the appearance of 

 Flemming's paper ('86), in which the author entirely confirmed the state- 

 ment previously made by Graf Spee ('84), that the system was of ecto- 

 dermal origin. This view was gladly welcomed on many sides, for it 

 was felt that an origin from this source was more in harmony with gen- 

 eral conclusions already accepted than was the method previously advo- 

 cated. Moreover, a new light seemed now to be cast on the phylogeny 

 of Vertebrates. Under these circumstances, it appeared highly desirable 

 that the position which Graf Spee and Flemming had taken be subjected 

 to the test of renewed investigation on other groups of Vertebrates than 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L. Mark, No. XXVII. 

 vol. xxi. — no. 5. 



