MORPHOLOGY OF CYCLOSTOMATA. 397 



The Anlage of tlie pronepliros is laid in exactly tlie same 

 manner from the fourth somite to the cloacal region. Under the 

 anterior three somites from the fourth to tlie sixth, the Anlagen 

 are developed into tlie pronephric chamber ; the Anlagen posterior 

 to these are all transformed into the " canal excréteur," as they 

 call the segmental duct, and they have come to the conclusion 

 that the '' canal excréteur " of the pronei)hros has the morpho- 

 logical value of a rudimentär}^ pronephric chamber. 



Tlie facts given in the last two papers, are thus in close 

 accordance with one another as well as with those given by 

 myself in the foregoing pages. Differences between their 

 results and mine are that the authors derive the system from the 

 lateral unsegmented mesoblast, and that both the parietal and 

 visceral layers of it partake in the formation of the system. As 

 has been stated in the descriptive part, this derivation is only 

 apparent ; a little further study shows that only the parietal layer 

 gives rise to the system, and this part of the layer belongs to 

 the somite. Indeed, this part appears to form, for some time, the 

 proximal portion of the lateral plate, being early cut otf from 

 the rest of the somite. It must be remembered that this separa- 

 tion is not the separation of the lateral plate from the somite, 

 but that of the Anlage of the pronephros from the rest of the 

 somite; or, the result of the development of the pronephros. It 

 is merely for a physiological reason that this development or 

 separation of the pronephric Anlage goes on earlier than, for 

 instance, in Selachia, it performing in Teleostei the actual ex- 

 cretory function. This will be understood easily, when a com- 

 parison with other groups is made further on. 



It has been a well known fact that the development of 



