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.