450 TREADWELL. [Vol. XVII. 



theory that this larval mesoblast (or, as he calls it, "ectomeso- 

 blast," because of its origin from ectoderm) is a distinct tissue 

 from the definitive "entomesoblast " (so called because it arises 

 from entoderm), and is homologous with the mesenchyme of 

 the turbellarian ancestors of the annelids, while the mesoderm 

 from which the adult structures arise is phylogenetically younger 

 and is represented prophetically in the ontogeny of such a form 

 as Discocoelis by the peculiar bilateral division of the cell 4d. 

 The ectomesoblast and entomesoblast are therefore phylogenet- 

 ically of different origin, a point previously urged by Meyer 

 (No. 23). 



Professor Wilson has certainly made out an attractive theory, 

 and the history of the two sorts of mesoderm in Podarke, espe- 

 cially the long-continued connection of the mesoblast with the 

 entoderm, at first sight lends confirmation to his view. For 

 reasons given below, however, I am not inclined to accept it as 

 stated, and propose instead a suggestion which seems to me 

 more in accordance with the facts. 



I believe that no hard-and-fast distinction can be made 

 between the two kinds of mesoblast. This belief is based 

 on the following facts : 



In Nereis, Wilson (No. 34, d) has shown that cells from the 

 anterior ends of the germ bands early separate from the bands 

 and pass forward into the segmentation cavity, where they 

 give rise to the larval musculature, which corresponds exactly 

 in structure and function with the larval mesoblast of Unio 

 or Podarke. 



In Lumbricus, also, Wilson (No. 34, a) has shown that 

 previous writers were in error in supposing an origin of 

 mesenchyme distinct from mesoderm, and that the mesen- 

 chyme of the embryo . is really formed from the anterior 

 cells of the germ bands which have migrated forward and 

 taken on a mesenchyme structure. Similar observations 

 have been recorded by Hatschek (No. 11) and Von Drasche 

 (No. 7). (See, however, p. 428.) 



To these observations must be added those of Wilson on 

 Hydroides (No. 34, b) : " In its earliest recognizable condition 

 the mesoblast band consists of a group of three or four cells, 



