4o6 WILSON. [Vol. VI. 



afterwards become pigmented {see figures of later stages) and are 

 finally pushed into the interior to form a part of the mesoblast, 

 the remaining portion being formed from the residue of the pri- 

 mary inesoblasts. The tnesoblast, therefore, arises from the poste- 

 rior lip of the blastopore. 



Meanwhile the first somatoblast undergoes an extraordinary 

 series of the divisions, quite constant in form and order, which 

 give rise to the cells of the ventral plate. The cells thus 

 formed grow forward ajid cover in the primary me sob lasts, com- 

 ing into juxtaposition with the small cells {bndded from the pri- 

 mary mesoblast) that form the posterior lip of the blastopore, and 

 that afterwards give rise to the pigment-area. 



Thus the ventral region consists of two distinct portions, viz. 

 (i) an anterior portion derived from the primary mesoblasts, 

 and (2) a much larger posterior portion, the ventral plate proper, 

 derived from the products of the first somatoblast. After the 

 mesoblasts have budded several times at the surface and are 

 entirely covered in, the character of their division changes, 

 each divides into two somewhat unequal parts (Figs. 61, 6"^, of 

 which the lower (mesial) is somewhat the larger. By continued 

 divisions (Figs. 79-82) the two cells on each side give rise to 

 the two mesoblast-bands, which are continuous below with the 

 superficial cells of the pigment-area (Fig. 70). Ultijnately the 

 pigment-cells themselves migrate inwards, and give rise, in part 

 at least, to the splanchnic mesoblast (Figs. 89-91). The longi- 

 tudinal muscles arise from the mesoblast-bands ; i.e. from the 

 p7'oducts of the primary mesoblasts after the superficial cells {pig- 

 fnent-cells) have been budded forth} 



^ This account of the mesoblast differs somewhat from that contained in my pre- 

 liminary paper, since I at first failed to observe the superficial budding of the primary 

 mesoblasts, and described the latter as only sinking below the surface after their 

 first division into two. This error of detail does not, however, affect the general 

 significance of the phenomena. I may add, at this point, that the essential point 

 of difference between v. Wistinghausen's account and my own is that he believes 

 some of the small cells budded forth from the primary mesoblasts to be true ectoblast- 

 cells which remain at the surface, whereas I believe that all migrate inwards, and are, 

 therefore, mesoblast sensu stricto. It appears to me that v. Wistinghausen has been 

 unduly influenced by Kleinenberg's views on this point, and I believe that N. Dum- 

 erilii is a far less favorable object for the investigation of the question, on account of 

 the lack of the pigment. My own studies are, however, not yet concluded, and I 

 hope to study other forms before publishing further accounts of the facts in Nereis. 



