6 WILSON. 



ing the fate of the small cells, however, my first account was 

 wide of the mark ; for I believed that they migrated into the 

 interior and spread out upon the walls of the archenteron to 

 form a part of the splanchnic mesoblast.^ I accordingly called 

 the small cells ''secondary mesoblast " and applied the same 

 term to the rudimentary cells of Aricia and Spio. Later studies 

 by several observers seemed to confirm this conclusion. Lillie 

 found in Uiiio a single pair of small superficial cells, budded 

 forth from the "primary mesoblasts " exactly as in Aricia or 

 Nereis^ but relatively larger, which he likewise believed to 

 wander into the cleavage-cavity to form a part of the mesoblast.^ 

 Heymons found in Umbrella two pairs of corresponding but 

 still larger cells, which he, too, apparently traced into the meso- 

 blast.^ Mead found that a corresponding pair of minute cells, 

 in Antphitrite are carried in at the tips of the mesoblast-bands ;* 

 while Holmes still more recently states that in Planorbis they 

 enter the segmentation cavity.^ Wierzej ski's recent observations 

 on Physa,^ though differing from the foregoing in some impor- 

 tant details, agree in referring the small cells, of which several 

 pairs are formed, to the mesoblast. With such an array of 

 confirmatory evidence my original conclusion seemed to be 

 strongly supported. Conklin, however, in his remarkable paper 

 on Crcpidida, reached a wholly different result, finding in that 

 gasteropod that cells which probably correspond with the small 

 cells of Nereis^ give rise to the posterior part of the archenteronj 

 In regard to Nereis, I have long suspected that my original 

 account of the fate of the small cells was erroneous. A re- 

 newed examination of the matter has left no doubt that such 

 was the case, and gives the strongest ground for the conclusion 

 that, like the corresponding cells in Crepidula, they enter into 

 the formation of the archenteron. The evidence for this con- 

 clusion is as follows : 



In my earlier paper on Nereis I overlooked the fact that, be- 

 sides the small cells derived from the " primary mesoblasts," 



"^ Nereis, p. 413. ^1897, p. loi. 



21895, P- 28. 61897, p. 389. 



31893, P- 281. "1897, p. 71. 

 < 1897, p. 248. 



