40 T. H. BüRLEND, 



oft* from the general coelom in some places, it becomes difficult to 

 say witli certainty" whether a tubule is pronephric or mesonephric. 

 This difficulty arises not only because of the slight gradational change 

 from posterior pronephric to anterior mesonephric tubule, but also 

 from the fact that all the pronephric tubules are not contempora- 

 neous, and that long before the last pronephric tubule is clearly 

 difterentiated, the first mesonephric tubules are also almost as far 

 advanced in development. 



There is no indication of a shifting backwards of the tubules. 



As is to be expected in a structure like the pronephros — 

 where degeneration is occurring — there is a certain amount of 

 variation of the kidney tubules in individuals of similar age; in no 

 case could I distinguish a metameric arrangement with one or 

 two tubules to each segment, but there were many cases in which 

 tubules were situated intersegmentally. However, the later embryos 

 show a closer approximation to metamerism (one tubule to each seg- 

 ment) than do the earlier ones, owing probably to the suppression of 

 intersegmental tubules. 



On account of the changes which take place in the nephrotome 

 region when metamerism becomes established in the anterior portion 

 of the bod}', the tubules occurring intersegmentally always open 

 into the coelom direct, and not into the lower end of the nephrotome. 



In segments VII, VIII and IX the tubules alwa3's open into 

 the coelom, although there are in segment IX indications of the 

 tubules situated in the middle of the segment tending to open into 

 the lower end of the nephrotome. In the earlier embryos in X 

 and in the later embryos in both X and XI the tubules more often 

 open into an open nephrotome or into a nephrotome which becomes 

 open at a later stage, than is the case in segment IX. 



In segments IX and X tubules with a single opening into the 

 pronephric duct often have more than one opening into the coelom 

 or nephrotome. We will leave the probable explanation of the con- 

 dition of the pronephric tubules in these embryos until we have 

 examined some later stages. 



Some later embryos which I examined, and which I distinguished 

 as 0, P, Q, R and S, have the mesouephros developed and much 

 more prominent than the pronephros. 



As I propose to deal with the mesonephros in Chrysemys in 

 another paper, I will describe the condition of the pronephros in 

 these embryos very briefly. 



