No. 3-] THE CELL-LINEAGE OF NEREIS. 43^ 



mesoblastic material, he denies that the two cells arising by its 

 fission are "primary mesoblasts," homologous with those of 

 Lunibriciis or Clepsine. 



A careful examination of v. Wistinghausen's descriptions and 

 figures has not enabled me to discover the evidence on which 

 this conclusion is based. His figures prove simply that the 

 "myoblasts" {i.e. the primary mesoblasts) sink below the surface, 

 as I have described. There is not the slightest indication of the 

 fate of the "untere Urzellen des Rumpfes." It appears to me 

 that V. Wistinghausen's conclusions have been influenced more 

 by the supposed theoretical demands of the views of Kleinen- 

 berg than by actual observation. I readily grant that my own 

 conclusions are not yet fully established. They rest, however, 

 on definite observations, which appear to be more complete and 

 satisfactory than those of v. Wistinghausen. 



It is difficult to take as serious v. Wistinghausen's criticism 

 on my comparison of the "posterior teloblasts " of Nereis to 

 those of the Hirudinea and Oligochaeta. It resolves itself 

 simply into this : that this comparison is inadmissible because 

 of the lack of distinct cell-rows in the ventral plate (!). It is not 

 necessary to waste space by replying to such a criticism. 



The divergence between v. Wistinghausen's results and my 

 own becomes more serious when the fate of the micromeres is 

 considered ; but it is clear that this is owing largely to the wide 

 and obvious gaps in his observations. The first set of micro- 

 meres (" encephaloblasts ") give rise, as he asserts, only to the 

 cephalic lobes (Kopflappen), from which are produced the cere- 

 bral ganglia and the cephalic sense-organs. The second and 

 third sets (the somatoblasts of course excepted) "are not con- 

 cerned in the formation of the body, inasmuch as they afford 

 no essential material for the formation of individual organs or 

 their parts." They give rise only to "the epidermis of the 

 annelid and an embryonic structure, namely, the pras-oral ciliated 

 belt " {I.e., p. 54). These conclusions so obviously rest upon 

 incomplete observations that they scarcely demand examina- 

 tion. It is clear from v. Wistinghausen's own figures that he 

 entirely missed the three spiral cleavages of the primary micro- 

 meres, and not one of the divisions of the other micromeres is 

 described or figured. His statements regarding the fate of the 

 micromeres, the origin of the prototroch, stomodaeum, etc., rest 



