MORPHOLOGY OF CYCLOSTOMATA. 403 



III the groups above referred to, the first indication of the 

 excretory system becomes apparent at a stage in which some 

 mesoblastic somites are formed and the metameric segmentation 

 of the mesoblast is going on. This is the Anlage not of the 

 segmental duct, l)ut oî the pronephros. A single exception is found 

 in Bdellostoiiia, in which the early traces of the system become 

 visible, as we learn from Price, at a stage much more advanced 

 than in other Anamuia, that is, at the stage in which the sclero- 

 myotome is cut off from the rest of the mesoblast and mesen- 

 chyraatous cells fill up the spaces between organs and organ-systems. 



I have endeavoured to reconcile the points, in which the 

 views of the previous authors diverge from one another, under 

 the following three headings : — 



A. — The Anlage of the Fronephr'ic Tubule is the Product 

 of the Ilesoblastic Soinîle and not of the Lateral Plate. 



The view that derives the pronephros from a single common 

 groove formed either of only the parietal, or of both the parietal 

 and visceral layers of the unsegmented mesoblast (the lateral 

 plate), is advocated by most of the authors who have worked 

 on the development of Petromyzon, Teleostei, and Amphibia. 

 This is due probably to the early separation of the sclero-myotome 

 from the rest of the mesoblast in these groups. In them the 

 Anlagen of the pronephros (or the nephric segments) together 

 with the lateral plate are cut ofï from the sclero-myotome and 

 form, for some time after this separation, the proximal portion 

 of the lateral plate. It must be borne in mind that this se- 

 paration is not the separation of the lateral plate, but of the 

 nephrotome, from the sclero-myotome. This is, therefore, a 



