66 T. H. BURLEND, 



development have made, in that she has regarded the degenerating- 

 traces of the anterior nephrotomes as incipient mesonephric tubules. 

 This mistake is pardonable when we recall the fact that a portion 

 of each mesonephric tubule is derived from the nephrotomy 



In Ganoids and Teleosts again the pronephros is stated to be 

 metameric. 



Thus there is abundant evidence of departure from the meta- 

 meric condition in the pronephros. What is the explanation of this? 

 The phenomenon of metamerism pervades the embryonic organs 

 from before backwards and from above downwards. It may extend 

 to the ventral middle line (Amphioxus) or extend down to the lateral 

 plate region only (other Vertebrates). At the same time the kidney 

 in the region below the nephrotome in the anterior region of the 

 body, and in the nephrotome region itself posteriorly, is developing 

 at a different rate. Does not the metamerism or non-metamerism of 

 any region of the pronephros depend upon its relative rate of growth 

 as compared with the rate of backward and downward progress of 

 metamerism ? Thus we should expect to find any region of the pro- 

 nephros not affected by segmentation which is differentiated before this 

 latter process, or conversely, if metamerism has made its impress felt 

 upon any region before the kidney is differentiated, the pronephros 

 will be segmented. 



Thus the anterior pronephric structures in Chrysemys tend to 

 exhibit less metamerism than do the posterior ones, because the 

 progress backwards of the phenomenon of metamerism is more rapid 

 than is the development and differentiation of the kidney posteriorly. 



Price found that the pronephros of Bdellostoma is non-segmental 

 anteriorly. 



In Selachii, Myxinoids etc. the mesonephric tubules are at first 

 metameric: this must naturally be the case since a portion of the 

 nephrotome — an undoubtedly metameric structure — takes part 

 in their formation. 



Owing to the fact that mesonephric tubules comprise a portion 

 of the nephrotome we find mesonephric tubules described as 1) formed 

 form retroperitoneal mesenchyme, Jungersen (1894), Felix (1897b), 

 2) from peritoneal ingrowths from the coelom, FükbeinctEr (1878), 

 ViALLETON (1890), Wheeler (1900). 



If the kidney formation (pronephric part) were phylogenetically 

 a later process than metamerism it is obvious that the typical 

 condition of the pronephros would be metameric. 



