454 TREADWELL. [Vol. XVII. 



he secured no evidence bearing on the primitive mode of 

 origin of this layer. In his later paper (No. 34, g) Professor 

 Wilson has suggested, as above stated, that the definitive 

 mesoblast came from entoderm, while the secondary is of 

 ectoblastic origin. The conclusion follows that the meso- 

 blast of the turbellarian is not homologous with the definitive 

 mesoblast of higher forms, but only with the larval. 



If we once assume that the mesoderm in different groups 

 must have exactly the same cellular origin in order to be 

 homologous, we land ourselves in an inextricable maze of 

 difficulties. Especially is this true if, in addition to the 

 strictly embryological data, we take into consideration the 

 facts of regeneration and bud development. The literature 

 of this subject is too familiar to need citing. (See, however, 

 Nos. 34, e, and 18.) Eisig (No. 8) devotes a considerable 

 amount of space to this question, and since any general 

 review of the various theories would be largely a repetition of 

 what he has said, I shall not attempt it here. Meyer (No. 23) 

 had previously maintained that the definitive mesoderm repre- 

 sents gonad tissue, while the mesenchyme had a different 

 origin. Eisig accepts this position not only for the first- 

 named tissue, but extends it so as to include the mesenchyme 

 as well, and supposes that both sets of mesoderm are really 

 gonad cells which may be mixed with any other cells without 

 losing their characteristic qualities or affecting the character 

 of the latter. From these may arise all the other cells of the 

 body, though in the actual ontogeny of the individual the 

 outer and inner layers having been provided for already, they 

 develop only into the middle layer. 



As said above, the accumulation of mesoblast at the posterior 

 end of the body seems to me a secondary condition, connected 

 with the need of supplying material for the elongating embryo, 

 and I see no evidence that it has now, or ever has had, a 

 gonad structure. The changes which must have occurred 

 since the layer was first differentiated are so great that it 

 does not seem to me we are justified in claiming positively 

 any especial mode of origin for it. 



We must, I think, go back to the position taken originally 



