MORPHOLOGY OF CYCLOSTOMATA. 385 



soon disappears, in Pelromyzon, without any definite significance. 

 At any rate, the structure represents " parts of the original 

 segmental coelome, that is, the nephrotoine," an unmistakable 

 fact which is denied by Price. 



The embryos of ^lyxine which formed the materials of the 

 valuable works by Maas are too old to be compared with those 

 of Petromyzon used in the present work. But the results ob- 

 tained by the author differ from those of Price in an important 

 point, namely, in the derivation of the mesonephros. 



The pronephros and mesonephros are, according to Price, 

 different parts of the same organ. " If the organ in question could 

 only be a pronephros alone, or mesonephros alone," says Price 

 '' I should unhesitatingly pronounce in favour of its being a pro- 

 nephros " [loco cit., p. 120). And he proposes to call " the 

 entire embryonic kidney holonepkro'<.''^ With Pabl, Maas, and 

 others, I hesitate to accept Price's conclusion ; for there are, 

 as may be inferred from his statements, great gaps not only 

 between the Stages B and C, but also between Stage C and the 

 adult. The formation of the mesonephros takes place in Petro- 

 myzon only at a stage much advanced, in which the processes of 

 the formation and degeneration of the pronephros go on in much 

 the same manner as in Bdellodoma, and it is open to doubt if the 

 mesonephros might not appear in later stages which were lacking 

 among Price's materials. 



Up to the oldest embryo observed by Price there were 

 neither glomeruli nor bloodvessels of a definite form, although there 

 were found in the splanchnopleure some vessels whose position 

 seemed to suggest their corresponding to the glomeruli of Selachia 

 and Amphioxus ; " but they do not have any relation to the 

 openings of the tubules, nor have they any direct connection 



