220 G. C. PRICE, 



the same to be true for the more posterior region. 4) In both stages 

 A and B, it is seen that the lumen of the duct develops in continuity 

 with the lumina of the tubules. This in itself is almost conclusive 

 proof that the entire system develops as a pronephros, and it also 

 indicates that the duct has been formed in continuity with the tubules, 

 for, if it had been formed independently, we would expect it to acquire 

 its duct independently also. 



As a further reason for interpreting the entire system as a pro- 

 nephros, may be mentioned the fact, that throughout almost its entire 

 length, certain coelomic pockets are formed, which are presumably 

 homologous with the pronephric chambers of Teleosts, Ganoids and 

 Ichthyophis. 



From all this, it would seem that the reasons for believing that 

 the entire excretory system of Bdellostoma develops in the same way 

 as does the pronephros of other Vertebrates, are but little less strong 

 than those for believing that the anterior end develops in this way, 

 and here we have demonstrable evidence. 



The series of embryos is sufficiently complete, so that it can be 

 said positively, that the tubules, which in the adult have been de- 

 scribed as mesonephric tubules, come directly from tubules which have 

 just been shown to arise as pronephric tubules. It can not be denied 

 that in the adult these tubules have essentially the same structure as 

 mesonephric tubules. This was first pointed out by Johannes Müller, 

 one of the most accurate observers that even lived, and since then, 

 so far as I know, it has been denied by no one, but on the contrary,, 

 the idea has met with universal acceptance, and a number of the 

 best morphologists have expressly stated their belief in its correctness. 

 In addition to its structure, the position of the organ in the body 

 would naturally point to its being a mesonephros. In structure, the 

 tubules in stage C resemble mesonephric tubules as much as in the 

 adult, and though the tubules themselves develop like pronephric 

 tubules, and the coelomic pockets like pronephric chambers, yet the 

 glomeruli, both in development and in adult structure, resemble the 

 glomeruli of a mesonephros much more than the glomus of a pro- 

 nephros. 



If the organ in question could only be a pronephros alone, or a 

 mesonephros alone, I should unhesitatingly pronounce in favor of its 

 being a pronephros; but it is possible that the difference between 

 pronephros and mesonephros is not a fundamental one, but that the 

 latter has been derived from the former, and that in Bdellostoma we 



