DEVELOPMENT OF NEPHRIDIA OF PHBRETIMA 



75 



dium from a stage when it consists of a mass of cells on the 

 septum (fig. 12 a) to a stage when it has attained to its adult 

 structure (Text-fig. 7). But in order to assign these septal 

 nephridia to one of the three primary germ-layers we must 

 trace the ultimate origin of this septal mass — the unformed 

 septal nephridium. We must note that an intersegmental 



Text-fig. 6. 



^ mm ^ %Q ee # H^ 



SJl 



in. 



Portion of the whole mount of an embryo showing septal nephridia 

 at a more advanced stage of development than those in fig. 13. 

 S.S., setal sacs ; other letters as above. 



septum is morphologically double and results from a coalescence 

 of the two layers of peritoneum covering the two faces of the 

 septum, and that although this double character of a septum 

 is not discernible in sections of an adult worm the two layers of 

 peritoneum can easily be distinguished in longitudinal sections 

 of embryos. The question naturally arises as to whether this 

 nephridial mass arises by a proliferation of one or more cells 

 belonging to the two layers of peritoneum forming the septum, 



^O. 261 G 



