1904.] NATURAL SCIENCES OF PHILADELPHIA. 361 



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Here the definitive mesoblast is said to arise from 3c and 3d, which 

 would be in correspondence with^Wilson's "ecto-mesoblast," while what 

 Eisig considers "larval" or "secondary"' mesoblast comes from that 

 portion of 4d which Wilson and Treadwell found in Nereis and Podarke 

 to form part of the wall of the enteron. These results have, it seems 

 justly, been called in question, though the careful investigation from 

 which they spring certainly gives credence to their accuracy. Tread- 

 well (1901) has called attention to certain figures (PI. XXXIX, fig. 42, 

 to PI. XL, fig. 49) of Hatschek on Eupomatus, which show "scattered 

 muscle cells in the upper hemisphere of the larva, which could hardly 

 have come from the feebly developed mesoderm bands at the posterior 

 end of the body", and suggests that they are of secondary origin; and 

 he likewise calls attention to the figures of Drasche (1884) for Pomato- 

 ceros which show similar conditions, though neither of these investiga- 

 tors appears to have realized their significance. In a preHminary paper 

 on the development of the mesoblast in Thalassema, Torrey (1902) 

 derives ecto-mesoblast from all three quartets. "In all there are 

 at least twenty primaiy cells of this character, but of them only ten, 

 arising from the first and third quartets, develop into functional mesen- 

 chyme, while at least ten degenerate and are finally absorbed by the 

 entoblasts." The greater part of the functional ecto-mesoblast comes 

 from three cells of the third quartet (3a, 3c and 3d) which correspond 

 closely to those which produce secondary mesoblast in Podarke. All 

 of the cells arising from the second quartet and which sink into the 

 segmentation cavity are rudimentary and in the end entirely degen- 

 erate, thus recalling AVilson's similar conclusions regarding the "rudi- 

 mentary" cells of the definitiA^e mesoblast of Aricia and Spio. At 

 least six derivatives of the seven ecto-mesoblast cells which Torrey 

 derives from the first quartet have a similar fate. 



The mesoderm of Platodes, Annelids and Mollusks has of late years 

 been subject to much studj^, and various theories have been propounded 

 regarding the significance of the manner of formation of the middle 

 germ layer of these groups. Without entering into a prolonged dis- 

 cussion with regard to this question, a few of the more general points 

 may be mentioned. The results above tabulated and my own observa- 

 tions lead to the conclusion — which is, of course, not here stated as new 

 — that the primitive mesoderm of these groups is represented by that 

 which arises from the ectoderm, and which is alone found in the Poly- 

 clad (Wilson). The suggestion of Wilson that upon this hypothesis 

 ecto-mesoblast might well be found arising from all three quartets of 

 ectomeres has just been verified by the work of Torrey, and shows that 



