228 TORREY 



a result of his extensive investigations of the post-larval devel- 

 opment of annelids, and by Wilson ('98) from a comparison of 

 the cell lineage of the mesoblast in platodes and annelids. 

 Although working from an entirely different point of view, both 

 of these authors have reached the conclusion that the ecto- 

 mesoblast (primary mesoblast, Meyer) and the coelomesoblast 

 (secondary mesoblast, Meyer) are phylogenetically entirely dis- 

 tinct. The former has been found to resemble closely in origin 

 (Lang, Wilson) and in fate (Meyer) the parenchyma of platodes, 

 while the latter is not represented as such in the flat-worms, but 

 is a later formation. 



The great body of embryological facts, it seems to me, is in 

 harmony with this view and the apparent exceptions emphasized 

 by Treadwell are not as serious as they may appear at first 

 sight. Although Wilson ('89) has described the outwandering 

 of cells from the anterior part of the mesoblast bands in Liun- 

 briciis and their probable contribution to the primary muscula- 

 ture, yet it is also very important to bear in mind that the outer 

 pair of teloblasts (ectoblastic) in the embryos of certain oligo- 

 chaetes and leeches have been found to give rise to the ring 

 musculature. Bergh ('90) and Vedovsky ('92) have found this 

 to be the case in Ltiuibricus, as has also Vedovsky ('90) in 

 RliyncJiebiiis and possibly Whitman ('87) in Clepsine. It is very 

 possible that in Nereis the true origin of the ectomesoblast has 

 been overlooked and the mesenchyme-cells, which Wilson 

 thought were differentiated from the coelomesoblast bands, may 

 have come secondarily into close association with them ; or, 

 again, the ectomesoblast may arise very late in the ontogeny, 

 as Meyer has found to be the case in a number of annelids. As 

 regards Hydroides, Treadwell calls attention to the fact that, 

 according to Wilson ('91), "the mesenchyme cells graduate 

 both in form and position into those of the germ bands " ; but 

 such is also the case in Thalassema, where their origin, never- 

 theless, is entirely distinct. Finally, I think it is generally 

 agreed that Eisig's unique description of the origin of the meso- 

 blasts in Capitella should not form the basis of wide generaliza- 

 tions until it has been confirmed in that and other forms. It is 



