The pronephros of Chrysemys marginata. 59 



Evidence of work on pronephros of Chrysemys in 

 relation to present views. 



The existence of such excellent summaries of the literature 

 dealing with the ontogeny of the Vertebrate kidney as those of 

 RÜCKERT (1891), Field (1891), Wheelee (1900), Felix (1906) and 

 the more recent work of Rabl (1908) renders it inexpedient for me 

 to attempt such a summary. I shall take the summary of Felix 

 and, without in any way desiring to deprecate his work or his 

 views, trust to show that his theoretical interpretation of kidney 

 development fails to explain conditions which undoubtedly exist in 

 the different vertebrate groups. 



I think we can gather from a careful study of Felix's views 

 that he has based them very largely upon the condition in Selachii, 

 Ganoids, and Gymnophiona, and that he has perhaps been imbued 

 with certain preconceived ideas by his researches upon the kidney 

 development in Teleosts. It is quite possible that the interpreta- 

 tion of the development in Gymnophiona is not correct, and that 

 it should be interpreted as I have done for Chrysemys. Again, there 

 is plenty of evidence to show that the conditions in Chrysemys are 

 not exceptional: some of this evidence I will attempt to adduce. 



Felix supposes that the pronephros is formed from the nephro- 

 tome (ürsegmentstiele) region of the body-somites; that metameric- 

 ally arranged evaginations (Hauptkanälchen) arise from the nephrotome 

 (one in each segment) ; and that these fuse at their outer (distal) ends 

 and so form a duct or "Sammelgang". Both the "Sammelgang" 

 and the tubules are included in the pronephros (Gesamtanlage der 

 Vorniere), which extends in Myxinoids, Teleosts, Ganoids (probably), 

 Amphibia (probably), Dipnoi and Petromyzontes along the whole 

 coelom with the exception of the extreme front end. 



In Selachii and Amniotes the pronephros is more or less shortened 

 and so another structure, the "freie Endabschnitt", is developed behind 

 the pronephros — in various ways — and so connects pronephros 

 with exterior. 



The „Hauptkanälchen" and the part of the nephrotome between 

 its proximal end and the coelom (Ergänzungskanälchen) together 

 form a complete pronephric tubule, the other portion of the nephro- 

 tome (between protovertebra and the opening of the Hauptkanäl- 

 chen into the nephrotome) atropliying sooner or later. 



Furthermore, the Ergänzungskanälchen may or may not become 



