410 s. Il ATTA : 



seen in the '* Nieren canal clien " would be brought about, if tlie 

 tubules in Pviroinyzon came to open to the exterior, boring 

 through the epiblast by the further growth of their free extremity. 

 This intimate contact of the tubule-end with the epiblast takes 

 place, as above mentioned, in the middle of Stage it, where 

 the tubules have developed a little beyond the mere Anlnge. 



The pronephric tubules of Feironiyzon are, in this stage, 

 already united with one another by the intersomitic solid cord ; 

 this union is, however, not primary, but secondary. This stage 

 presents, I think, the phylogenetic stage, in which the " Niereu- 

 canälchen" with separate external segmental openings, and the pro- 

 nephric tubules with the collecting duct, diverge from each other. 



By this assumption, it is not meant that in the ancestry of 

 Chordata the tubules were closed blindly inside the epiblast ; for 

 the Anlage of the pronephric tubule might have been, in the 

 ancestral form too, brought about by the folding of the mesoblast, 

 to break out finally to the exterior. This perforation would 

 become unnecessary when the secondary union of the tubules had 

 been acquired. 



Since a certain number of the '"' Nierencaniilchen " in front 

 of the base of the hepatic cu:;cum, is represented by the pronephric 

 tubules of the glandular part in Cyclostomata, those lying over 

 it will be homologous with the pronephric tubules which are 

 found over and posterior to the hepato-pnncreatic Anlnge and 

 converted into the anterior section of the segmental duct, being 

 secondarily united with one another by the confluence of the free 

 extremities of the tubules. 



There is not to be seen tlie post-hepatic " Nierencanälchen " 

 in Ainqyhioxus. We learn from Laxkestee ('89) and Willey ('91) 

 that in Amphioxus, the new branchial slits are added, by stages, 



