68 T. H. BURLEND, 



or the position in the body, of the posterior part of the pronephros 

 and the anterior part of the mesonephros. The mesonephros does 

 not overlap the pronephros, nor is there a space between the end 

 of the pronephros and the beginning of the mesonephros. The 

 posterior pronephric tubules unite secondarily with the pronephric 

 duct in exactly the same way as do all the mesonephric tubules, and 

 the only essential différences between a posterior pronephric and 

 an anterior mesonephric tubule are due to the fact that the former 

 develop in a region of the body which is not so appreciably changed 

 by the ''rotation" factor to be described later, as is the posterior 

 region of the body in which the mesonephric tubules develop. 



Thus we have the following structural gradations: — antr 

 pronephric tubules — with nephrostomes opening into the 

 general coelom laterally to the nephrotome: the glomus hangs freely 

 into the coelom. 



postr pronephric tubules — with nephrostomes opening 

 into the general coelom when situated in the region between two 

 segments, or into the open nephrotome when situated in the region 

 opposite a protovertebra. The nephrotome communicates with the 

 coelom, and the glomus now comes to hang into the nephrotome and 

 to show signs of subdivision. 



antr mesonephric tubules — the nephrostomes always 

 open into the shut off nephrotomes, the latter containing portions 

 of the divided-up glomus which tend to fuse with the nephrotome 

 wall in places and so form smaller chambers (Malpighian capsules), 

 each containing a glomerulus: into the capsules open at a later date 

 nephrostomes derived presumably from the subdivision of the original 

 nephrostomes. 



The condition in Chrysemys tends to prove that the pronephros 

 and mesonephros are not separate dyshomodynamous structures, but 

 merely differentiated yet homologous regions of the same organ, which 

 differ owing to their subjection to unlike environmental influences. 



That there is evidence of a close relation between pro- and 

 mesonephros is shown by the difficulty of distinguishing between 

 pro- and mesonephric tubules in many other Vertebrates. Thus it 

 has not been definitely decided whether the entire excretory system 

 of the adult Myxine is pronephric onlj^, or whether the posterior 

 region is mesonephric. From the structure of the posterior tubules 

 one would be inclined to speak of them as mesonephric — in which 

 case Pkice says there is no difference" in the development of what 



