8 WILSON. 



their nuclei. The first of them to be formed are budded forth 

 at the surface near the lower pole at a time when the ''pri- 

 mary mesoblasts " have budded three or four times (Fig. 3, D). 

 Those produced later do not reach the surface, the macro- 

 mere-nuclei receding from the surface and leaving below them 

 (towards the surface) a closely packed mass or plug of small 

 cells (Fig. 3, E), the more anterior of which have been de- 

 rived from the macromeres, and, therefore, are unquestionably 

 of entoblastic origin,^ while the more posterior have been 

 derived from the "primary mesoblasts." This plug is bor- 

 dered in front and at the sides by the ectoblast-cells of the lips 

 of the blastopore, which has now become much diminished in 

 size, while posteriorly it abuts superficially against the ecto- 

 blast-cells of the somatic plate (derivatives of " d^ " or '* X," the 

 first somatoblast) and at a deeper level against the primary 

 mesoblasts (Fig. 3, E). In the cells of this plug are now de- 

 veloped coarse granules of black pigment (Fig. 3, Z'), by means 

 of which they are so unmistakably marked that their later his- 

 tory may be followed step by step with great accuracy. Thus 

 arises the pigment-area at the lower pole of the trochophore 

 larva, described in my first paper on Nereis? 



In that paper I concluded that the pigment-cells were derived 

 sotely from the " primary mesoblasts," having overlooked the 

 fact described above that a part of them, and probably the 

 greater part, are derived from the macromeres (entomeres). I 

 reached the further conclusion that the pigment-cells wandered 

 into the interior and spread out upon the wall of the archenteron 

 to form a part of the splanchnic mesoblast.^ Renewed studies 

 demonstrate the erroneous nature of this latter conclusion, and 

 prove that the pigment-cells give rise to the posterior part of the 

 arcJienteric zvall itself. Both in total preparations and in serial 

 longitudinal sections * of the successive stages, every step can 



1 These cells are obviously comparable to the entoblast-cells of the fourth and 

 fifth quartets (and later entoblast-derivatives) in other annelids. In iVereis they 

 show no definite arrangement. 



21892, pp.412, 417. 



^Nereis, p. 413. 



4 The best results were obtained with strong Flemming's fluid. 



