No. 3.] PODARKE OBSCURA VERRILL. 453 



posterior end of the body is also a familiar feature of annelid 

 ontogeny. 



Now, as there is no need to assume a phylogenetically new 

 formation of ectoderm for the body as distinct from the head, 

 so I see no need for assuming a new formation of mesoderm. 

 This is evident in Nereis or Lumbricus, and I see no reason why 

 a difference should be made in the case of Aricia or Podarke. 

 In the former case there has simply been a more complete 

 concentration of mesoderm than in the latter. If the facts 

 bear on the point at all, they tend to show an origin of meso- 

 derm from ectoderm rather than from entoderm, though this 

 concentration of material must have led to such changes that 

 we are hardly justified, from the present position of the meso- 

 derm, in drawing any conclusions as to its primary mode of 

 origin. 



I believe, therefore, that the mesoderm cells collected at the 

 posterior end of the trochophore larva, and which we know as 

 the definitive mesoblast, represent neither entodermal evagi- 

 nations nor gonad tissue, but that they represent the meso- 

 derm of the body, which is morphologically continuous with 

 that of the head (as we know it is in Nereis or Lumbricus), and 

 which has been concentrated at this point to provide for the 

 needs of the elongating body. In some cases this concentra- 

 tion is less complete than in others, so that some forms seem 

 to have a double origin of mesoblast, but this (apparent) double 

 origin in no way interferes with the morphological unity of 

 the tissue. Whether the mesoderm arose originally from ecto- 

 derm or from entoderm or from neither, we are not at present 

 in a position to say. 



Rabl's theory of the mesoderm (No. 27, c) seems to have 

 rested too much on an error of observation (Hatschek's de- 

 scription of the mesoblastic teloblasts in Amphioxus ; see also 

 Minot's criticism, No. 24, p. 155), and the " Coelomtheorie " of 

 the Hertwigs (No. 13) makes, as I believe, too sharp a dis- 

 tinction between mesoderm and mesenchyme. In the intro- 

 duction to his paper on Nereis, Wilson hopes, by a study of 

 cytogeny, to break away from the deadlock of opinion concern- 

 ing the mesoderm, but as I understand his conclusions on p. 393, 



