No. I.] GERM-LAYERS IN CLEPSFNE. 171 



tions to his idea of the origin of the definitive epidermis. Klein- 

 enberg adds (p. 130), that in all Polychaeta, whose develop- 

 ment is known to him, the epidermis of the larva is replaced by 

 the definitive, outer epithelium of the annelid ; but this takes 

 place through a process of transformation, which has its point 

 of departure in the larval epidermis itself, and only in a {q^n 

 cases are the parts of the old ectoderm actually thrown off. 



Is it not possible that the permanent epidermis in Aulostoma 

 and Nephelis has its origin in the larval epidermis? Such an 

 origin would accord perfectly with what takes place in other 

 annelids, and remove the apparent discrepancies in develop- 

 ment between the Rhynchobdellidas and the Gnathobdellidae. 

 Bergh's figures appear to lend no support to the suggestion; 

 but important points in the history of this neuro-nephric stra- 

 tum have escaped his attention, and a reexamination is required 

 in order to settle them. If it turns out that this stratum gives 

 origin to the nephridia and nerve-chain, as in Clepsine, the con- 

 formity in development between the two classes of leeches will 

 be settled beyond a doubt. 



5. Sig}iificaiice of the Teloblasts. 



The teloblasts form one of the most remarkable features of 

 annelid development. They represent specialized centres of 

 proliferation, with most marvellous powers of assimilation and 

 reproduction. Their occurrence in worms, molluscs, and verte- 

 brates (only mesoblastic teloblasts have thus far been dis- 

 covered outside the annelids), in larval as well as in foetal types 

 of development, makes it sufficiently evident that they are not 

 to be regarded as an accidental phenomenon without morpho- 

 logical significance. 



The embryos of all bilateral animals, from the worms up to 

 the vertebrates, lengthen by cell-proliferation at the posterior end. 

 The question arises. Is this proliferating power invariably local- 

 ized in special cells or groups of cells? It is generally believed 

 that the posterior end of the embryo represents a mass of indiffer- 

 ent, non-specialized elements. It is supposed that here histolog- 

 ical differentiation has its vanishing point ; that here the germ- 

 layers blend in a common basis. 



The case of Lumbricus {vide Wilson's paper) shows us that 



