The pronephros of Chrysemys marginata. 69 



become anteriorly the pronephric tubules of the adult, and of what 

 become the mesonephric tubules posteriorly. 



From considerations of the same nature Field was led to speak 

 of a "holonephros", and Roule & Audigé (1908) to speak of a 

 •'mononephros". 



Furthermore, it is quite easy to see that those animals with 

 mesonephric tubules having- persistent nephrotomic openings to the 

 coelom exhibit an intermediate condition between the true pronephric 

 and the true mesonephric condition, for they have the lower end of 

 the nephrotome open — viz. the nephrostomal canal, whilst the upper 

 swollen part of the persistent nephrotome becomes the Malpighian 

 body. 



This explains the condition met with in Selachii, Ganoids etc. 



The closure of the nephrostomes is effected by a fusion of the 

 vascular glomus with the somatic mesoderm at a place lateral to the 

 tubule openings, Wiedersheim foreshadowed this explanation in a 

 paper on the urinary development of Crocodilia and Chelonia: his 

 theoretical conclusions were not generally accepted as probable because 

 his earliest embryos were already too old for a thorough enquiry 

 into the development of the pronephros. I am quite satisfied that 

 Wiedersheim's embryos did show the pronephros, although it was 

 in its later stages. 



Assuming that we have accurately set forth the evolution of 

 the mesonephric condition from the more primitive pronephric the 

 explanation of outer pronephric chambers etc. becomes easy. Outer 

 pronephric chambers will occur only in the anterior region of the 

 coelom, where the nephrotome is very short and does not include 

 that part of the somatic mesoderm in which the pronephric nephro- 

 stomes are situated. So that the dorsal region of the coelom in 

 the front part of the pronephros is the homologue of the swollen 

 nephrotome regions or Malpighian capsules of the mesonephros. Both 

 tend to become shut off in further growth and specialization of 

 the coelom, the result being that the outer pronephric chambers 

 have in some cases narrow connections with the rest of the coelom 

 (Pseudonephrostomalkanälchen of Ganoids, Felix), whereas the homo- 

 logous regions in the mesonephros are represented by the nephro- 

 stomal canals. The posterior region of the pronephros in Chrysemys, 

 Ganoids aud Gymnophiona presents an intermediate condition between 

 that found in front and behind it: the nephrotome region has now 

 become elongated and swollen; the metamerism is only just beginning 



