SOME PROBLEMS OF ANNELID MORPHOLOGY. 6/ 



from neither directly, but being differentiated in the 

 course of cleavage. It is described as arising from a 

 single pair of pole-cells, from several pairs of pole-cells, 

 from no pole-cells at all — and so on through a long list 

 that might be given did space suffice. It is, in short, 

 simply impossible, at present, to reconcile the various 

 modes of mesoblast-formation in annelids as described 

 by various good authorities, and there is scarcely a 

 more confused subject in comparative embryology, or 

 one which more pressingly demands revision. It is no 

 wonder that Kleinenberg, who has been largely influ- 

 enced by the study of annelids, attempts to cut the 

 Gordian knot by denying the very existence of the 

 mesoblast as a "germ-layer" — ''es gibt gar kein Meso- 

 derm." Nevertheless very few forms have as yet been 

 adequately studied. Indeed, scarcely a single case has 

 been exhaustively worked out ; and while this is the 

 case, we need not despair of reducing the various modes 

 of mesoblast-formation to a common type. Until this 

 has been accomplished, however, it will, I believe, be 

 premature to speculate on the origin and meaning of 

 metamerism. 



We may come now to closer quarters with the prob- 

 lem of concrescence and the meaning of the trochophore, 

 questions that have thus far only been alluded to in 

 passing. I am obliged to treat this part of the subject 

 in a more technical manner, though I fear it will be at 

 some sacrifice of intelligibility to those not especially 

 interested in the subject. If we examine the embryo of 

 a leech {Clepsine), in the middle period of development 

 (Fig. 4, A), we find that the future alimentary canal is 

 represented by three macromeres (yolk-cells), distended 



