4 WILLIAM MORTON WHEELER, 



carrier that enabled me to cut wedge-shaped sections. This method, 

 which is rather slow, is not necessary after stage 3, as the longi- 

 tudinal axis of the embryos and larvte is thenceforth perfectly straight. 



Part I. Historical. 



For a general account of the extensive litemture on the excretory 

 organs of Vertebrates the reader is referred to the admirable sum- 

 maries and bibliographies of Field (1891) and Rückert (1891). In 

 this place I shall consider only the literature relating to the urino- 

 genital organs of Fetromyzon. 



1. The Pronephros. 



Rathke (1827, p. 99) seems to have been the first to describe 

 the excretory organs of Ammocœtes, so far as they could be studied 

 under a low magnification. He saw the pigmented pronephros on 

 either side just above the heart, and noted the fold that runs back 

 and connects it with the mesonephros of the same side. On the sur- 

 faces of the pronephroi he detected "einige (ungefähr 10 — 12) drüsen- 

 artige, kleine, weissgefärbte , und mit einem kurzen Stiele versehene 

 Körperchen , welche bei 2 der von mir untersuchten Exemplare an 

 ihrem dickern Ende becherförmig, jedoch nur wenig, ausgehöhlt, bei 

 den übrigen aber abgerundet waren". These structures, which Rathke 

 at first took tj be hydatids are evidently the pronephric funnels. The 

 number, ten to twelve, refers to both pronephroi as shown in his fig. 8, 

 tab. 2, He seems also to have seen the convolutions of the pronephric 

 tubules through their pigmented epithelial covering, but to have 

 mistaken them for spherical bodies. 



Max Schultze (1856, p. 30) saw the pronephros in the embryo 

 lamprey viewed as a transparent object. He detected the movements 

 of cilia in a region which he interpreted as a groove, but which was 

 evidently the openings of the pronephric funnels. He also remarked 

 that the pronephros corresponded in position to the Wolffian body 

 of fishes but failed to make out its connection with the kidneys. 



Wilhelm Mi^ller (1875) was the first to give a good account 

 of the structure and development of the urinogenital organs of Fetro- 

 myzon. The earliest stage of the pronephros observed was in an 

 embryo of P. fluviatilis with four gill clefts. Just behind the heart 

 he found "in der seitlichen Wand der längs des Pharynx nach vorn 

 sich erstreckenden Peritonealhöhle beiderseits eine runde OeÖ'nung, 



