The pronephros of Chrysemys marginata. 53 



3. With the degeneration of the pronephros the degeneration 

 of the aortic branches occurs pari passu. 



4. The development and atrophy of the blood-supply to the pro- 

 nephros is retarded on the right side as compared with the left side. 



5. There is evidence that in its earlier stages the blood-supply 

 to the pronephros is in the form of a continuous structure on each 

 side of the aorta. 



Textfig. C consists of a series of diagrams drawn with the 

 aid of a camera lucida from successive sections of the anterior 

 end of the left kidney of embryo P. The gradual change in position 

 of the vascular structure supplying the kidney as one passes from 

 pronephros to mesonephros is interesting; also the subdivision of 

 the vascular tissue into branches, and the preliminary subdivision 

 of the coelom in the region around the vascular tissue by fusion of 

 vascular branches with the somatic mesoderm in regions lateral to 

 the nephrostomic openings of the tubules. Thus one may roughly 

 distinguish: 



a) an anterior region where the glomus is external and freely 

 projecting into the coelom (Textfig. C 1—17). 



(cf. the condition in Petromyzon Wheeler), 



b) a middle region in the postr pronephric region (Textfig. C 

 18 — 35), where the subdivided glomus shows internal and external 

 glomeruli, and 



c) the mesonephric region posteriorly, where the vascular tissue 

 is becoming subdivided into internal glomeruli only (Textfig. C 

 36-50). 



A strong argument in favour of the transition above mentioned 

 is that in no Vertebrate embryo where so-called "internal" and 

 "external" glomeruli occur do we find the external glomeruli 

 extending as far back as the internal glomeruli do. In fact it is 

 only in the cases where pronephros passes into mesonephros without 

 break that "inner pronephric chambers" and consequently "internal 

 glomeruli" have been described (viz. Myxinoids, Teleosts, Ganoids, 

 ? Gymnophiona). 



It seems obvious that the primitively non-metameric glomus will 

 become afi'ected by metamerism the more dorsal (relatively to the 

 coelom) it becomes, provided it experiences an "arrested" development 

 in the mesonephric region — and this is the case in Chrysemys and 

 many other animals. 



