332 



RANDOLPH. [Vol. VII. 



and indifferent tissue, which is later differentiated into ectoderm 

 and entoderm. 



Von Kennel argues (9) that while the homology of the germ 

 layers of Vertebrates is established beyond doubt, among Inver- 

 tebrates it does not hold. This conclusion is based upon the 

 observation that in Ctenodrilus a considerable part of the pos- 

 terior region of the alimentary canal is proctodeum ; and that, 

 therefore, when a new zooid is formed in this region of the 

 worm, the lining of its alimentary tract is ectodermic in origin, 

 and hence cannot be homologized with the entodermic canal 

 of the individual produced from the q%^. 



Semper (23) in the budding of the Naids and Chaetogaster 

 derived the mesoderm of the bud from ectodermic cells.^ 



In the case of Antedon referred to in the introduction, it 

 seems possible that some of the entodermic tissue may remain 

 after the ejection of the visceral mass. If only a very small 

 part remains, the regeneration of the visceral mass is not an 

 unparalleled case. 



Lastly, Biilow's results were opposed to the theory of the 

 homology of the germ layers. 



My results, however, are in sharp contrast to those of Biilow 

 in regard to the origin of the new mesoderm. My observations 

 show that each germ layer gives rise to the corresponding new 

 tissue in the regenerated part, — or, in other words, that the 

 structures of the newly formed somites may be traced to the 

 ectoderm, mesoderm, and entoderm, respectively, of the devel- 

 oped part of the worm, just as these arose from the respective 

 germ layers of the embryo. 



The method of regeneration in Lumbriculus seems to be in 

 harmony with the proposition stated by Balfour (i) in regard 

 to the mesoblast, that with its differentiation as a distinct layer 

 the two primary layers lost for the most part the capacity they 

 primitively possessed of giving rise to structures now develop- 

 ing from the mesoblast. 



On the other hand, the existence of a third germ layer is not 

 admitted by some investigators. In this connection Kleinen- 



^ The presence of neoblasts in these forms has already been alluded to. The 

 large contributions from the ectoderm are probably to be regarded as the condensa- 

 tion of the lateral foundations that arise in the ectoderm and subsequently come to 

 lie more centrally. 



