MORPHOLOGY OF CYCLOSTOMATA. 381 



fact that the authors probably overlooked the earliest phases of 

 foruiation, which take place, as shown above, in a stage very 

 young, but not youuger in comparison than that in other Anarania ; 

 for the formation follows the metamerio î<cgiii<niation of the uuxohlast 

 ill the anterior i-ogion. Tn later stages, tlio tu1)nles and the inter- 

 soinitic portion of the collecting duct repeated in sections of a 

 series appear, indeed, lila^ tlie cross-sections of a longitudinal 

 furrow or groove of the lateral plate, the lips of which are fused 

 at certain points, as described by Shipley and Goette (see my 

 figs. 66-74). 



The number of the tubules and nephrostomes varies accord- 

 ing to the stages of development. And if some stage or stages 

 are overlooked, it must necessarily lead to an erroneous conclu- 

 sion. This is the probable reason why the statements of the 

 writers with reference to the number differ. 



Indeed, the anterior extremity of the pronephros has already, 

 from the first appearance, the features of a rudimentary organ ; the 

 first pair of the tubules can not be observed at the same time 

 with the following five pairs, except by extremely good luck. In 

 some embryos of Stage in, we see occasionally the collecting duct 

 alone in front of the first tubule, so that we are led to infer 

 that there were some pairs of tubules in front of the present 

 first pair, which have degenerated during the course of tlie 

 ancestral history. ^^ 



As is seen above, all investigators who have been occupied 

 with the study of the development of Petromyzon agree in de- 

 scribing only one pair of glomeruli. Shipley says ''there is only 

 one glomerulus on each side, stretching on each side of the 



1) I have stated above that in the earliest part of Stage iii, the anterior extremity of 

 the left collecting duct presents a conical protuberance (see the footnote on p. ,'Î29)- 



