The pronephros of Chrysemys marginata. 73 



sich auch die Lagebeziehung des primären Harnleiters zum Ur- 

 segmentstiel. Solange er als Ausführungsgang der Vorniere funktio- 

 nierte, lag er unmittelbar lateral vom Ursegmentstiel , nach der 

 Drehung wird er von demselben überlagert und liegt jetzt an der 

 ventralen Seite derselben." 



Felix finds this process going on in the developing Trout 

 (Forelle). Speaking of the developing pronephros he says: 



"Später verschieben sich dorsaler und ventraler Abschnitt, der 

 dorsale wandert lateralwärts , der ventrale raedianwärts, so daß 

 beide Abschnitte nebeneinander zu liegen kommen und das Pseudo- 

 vornierenkanälchen nicht mehr abwärts, sondern medianwärts um- 

 biegen muß, um die Vornierenkammer zu erreichen." What Felix 

 speaks of as the "'inner pronephric chamber" is the swollen lower 

 end of the nephrotomy 



Before leaving this "rotation" factor we must add that the 

 amount of rotation is not uniform in each body-segment. So far as 

 I am able to judge at present it is at the front and hind ends of the 

 segment that the rotation is least, and in the middle of each 

 segment that the rotation is greatest. This statement is tentative, 

 since I have not as yet pursued this matter very far, and so I leave 

 it for the present sub judice. 



This unequal rotation in each segment I have observed in the 

 pronephros of SctjUium and Chrysemys: if it is universally true, it 

 explains many statements that the supporters of the Sedgwick-Field 

 theory were unable to account for. 



The result of this uneven rotation in the body-segments con- 

 taining the posterior part of the pronephros is, that whereas any 

 tubules opening at the front or hind end of the segment would open 

 direct to the coelom and so be typically pronephric, the middle 

 tubules in the same segment would open into the nephrotome and 

 so appear to be mesonephric. Thus pronephros and mesonephros 

 might appear to overlap, e. g. Pristiurus (Rabl 1896) and Chelonia 

 (Gregoey 1900) etc. It also may explain the statement that when 

 pronephric and mesonephric tubules appear in the same segment 

 the former are anterior to the latter. 



3. The effect of metamerism on the developing 

 kidney. The results of metamerism in any animal will depend 

 upon : 



a) Whether the process of metamerism starts before, at the 

 same time as, or after the process of kidney growth. 



