214 G. C. PRICE, 



difference in the system in these regions. Throughout both regions, 

 and also for two or three segments still cranialwards , the system is 

 in essentially the same stage of development. 



Should a detailed description of the entire excretory system in 

 stage B be given, much of it would be a tedious repetition of what 

 has already been given. But there would be this important difference ; 

 the system develops from behind forwards, and not in the opposite 

 direction, as one would expect, so that a description which would 

 apply to a certain region in stage A, would apply, not to the same 

 region , but to one more anterior in stage B. For this reason the 

 work of description will be materially lightened. 



For stage B there are two embryos, but in one the excretory 

 system is somewhat further advanced than in the other. In the 

 youngest, which will first be described, there is a rudimentary tubule 

 on the left side in the sections passing through the sixth spinal 

 ganglion, and on the right, there is one in the sections passing between 

 the eighth and ninth spinal ganglia. Thus it will be seen that the 

 system has advanced cranialwards through five segments on the one 

 side, and through two and a half segments on the other, for in stage 

 A it ended on both sides in the eleventh segment. What are spoken 

 of as rudimentary tubules, are simply solid knobs of cells, which are 

 attached to the coelomic epithelium, and in structure differ from the 

 anterior tubule anläge in stage A, but they resemble it in being un- 

 connected with the following part of the system. On the left side 

 there is a second such independent rudiment just back of the first. 

 It is quite likely these would have disappeared without having formed 

 actual tubules, or without having been connected with the rest of 

 the system. 



Throughout the eighth, ninth, tenth and eleventh segments on 

 the left side, the segmental duct is still connected with the coelomic 

 epithelium, although in some sections it is almost constricted oft'. In 

 the twelfth segment, and from here on , it is entirely free from the 

 coelomic epithelium. The process of canalization in this embryo is 

 almost diagramatically regular. Between the first actual tubule and 

 the second the duct is entirely solid, but the lumen of the second 

 tubule extends a short distance caudalwards into the duct, and from 

 here on the process is repeated, the lumina extending farther and 

 farther into the duct, and always caudalwards, although in a few cases 

 they may also extend a very short distance cranialwards. In the 

 region corresponding to the pronephros and mesonephros of the adult, 



