No. I.] GERM-LAYERS IN CLEPSINE. 169 



cases ; and no artificial lines of distinction, such as we are ac- 

 customed to draw between the germ-layers, can lessen its sig- 

 nificance. When our definitions of the germ-layers fail us we 

 must appeal to the precise genealogy of the cells. To deny the 

 existence of a mesoderm is of no avail ; for, with two primary 

 layers, — ectoderm and entoderm, — we are just as far from 

 being able to settle the question of morphological identity. 

 When, as in the case under consideration, we find an organ 

 arising sometimes from the ectoderm, and at other times 

 from the mesoderm, we have to admit that there is no fixed 

 and impassable boundary-line between these two layers ; and 

 that its association with this or that germ-layer is not an infal- 

 lible guide to its morphological identity. 



The following view ofifers a fair explanation of the point in 

 question : Both the mesoblasts and the nephroblasts arose 

 primarily from a common ectodermic basis. The genetic rela- 

 tions of the two cells have remained essentially the same ; but 

 the time of their difi*erentiation as distinct cells varies. If the 

 division takes place within the ectoderm, then each makes its 

 exit from the original seat separately and independently of the 

 other; if, on the other hand, the division is delayed until after 

 the separation from the ectoderm is accomplished, then the 

 nephroblast appears to arise from the same source as the meso- 

 blastic bands, and thus to form a part of these bands. The dif- 

 ferences noted between Eupomatus and Clepsine may be recon- 

 ciled in this way. The conflicting accounts given of the origin 

 of the vertebrate " segmental duct " admit of a similar ex- 

 planation. 



There are, then, some very positive indications that the larval 

 nephridium consisted, originally, of a single cell; and the gen- 

 eral occurrence of nephroblasts, as the basis of both systems of 

 organs, is in favor of this view.^ 



4. Tlie Origin of the Epidermis. 



Bergh has shown that it is necessary to distinguish between 

 the larval and the definitive epidermis of the Gnathobdellidae. 



' I am reminded of the opinion long ago expressed by Leuckart (No. 1 7, pp. 698— 

 699), that the teloblasts of Clepsine (" Colossale Zellen ") represent " Urnieren." 

 Leuckart supposed, however, that they were provided with ducts, and that they were 

 functionally active. 



