8o Jenkinson, Germinal Layers of Vertebrates. 



dermal origin is formed over the cut surface. This 

 becomes covered by an overgrowing layer of epidermiSj 

 and subsequently perforated by the forward (or backward 

 in the case of the regeneration of the tail) growth of the 

 blind end of the gut ; after the fusion of this with the 

 epidermis a new opening — mouth or anus — is effected. 

 It will be noticed that in this case the original stomodaeum 

 and proctodaeum are replaced by endodermal epithelia. 

 Haase's^^ description of the processes that occur in 

 Tubifex is very similar. MicheF', on the other hand, 

 states for a very large number of Oligochaeta and Poly- 

 chaeta — and von Wagner^^ {Lnmbriculus) and Hepke^^ 

 {Nats) are at one with him in this matter — that the cap of 

 reparation tissue which developes over the wound is due 

 largely if not entirely to an active proliferation of the 

 epidermis. All three observers agree in deriving not only 

 the new nerve-cord but also the new mesoderm — including 

 the ' ectodermal ' nephridia — from this mass of cells ; but 

 differ in their description of the regeneration of the gut. 

 According to Michel the intestinal aperture made by the 

 cut never closes, but persists as the anus. Von Wagner 

 also derives the lining of the regenerated portions from 

 that of the old alimentary tract ; the gut closes first and 

 then grows through the reparation tissue to fuse with the 

 epidermis and open to the exterior ; there is only a very 

 slight invagination at the aperture. Hepke, on the con- 

 trary, describes the forward (or backward) growth of a 

 solid cord of cells derived from the epidermal reparation 

 tissue which, acquiring a lumen, opens into the old gut at 

 one end and to the exterior at the other. 



^^ Haase, H., Zeiischr.zviss. ZooL, vol. 65, p. 211-256, 1899. 



** Bull. Sci. de la France et de la Belgigtie, vol. 31, p. 245 — 401, 1898. 



5 2 Zool.Jahrb. {Anat,), vol. 13, p. 603-682, 1900. 



5* Zeitschr. wiss. ZooL, vol. 63, p. 263-291, 1898. 



