No. 3-] THE CELL-LINEAGE OF NEREIS. 413 



I have made many actual sections through the pigment-area 

 which clearly show the structure of the mesoblast-bands, but I 

 figure an optical section (Fig. 70), since it shows much better 

 the relations of the parts. The sections show that the cells of 

 the mesoblast-bands ijn.b) are perfectly continuous with those of 

 the pigment-area, from which they are no longer to be distin- 

 guished in size. The pigment-cells extend to the surface and 

 might be described with Salensky or Kleinenberg as forming 

 a part of the ectoblast. They are, however, sharply differenti- 

 ated from the cells of the ventral plate iv.p), not only by the 

 presence of the pigment-granules, but also by a distinct line 

 of demarcation, and the cells do not take the color (Schneider's 

 acetic carmine) in quite the same degree. 



In this paper I shall treat very briefly of the later history of 

 the mesoblast, since I wish to leave this to a second paper. I 

 will therefore give only such a brief sketch of the leading facts 

 as will serve to connect the embryonic topography with the larval 

 and adult bodies. The pigment-area remains distinct until the 

 elongation of the body has made considerable progress (Figs. 

 85-89) and the mesenteron has acquired a small cavity. HThe 

 pigment-cells then migrate inwards (Figs. 90, 91) and spread 

 out upon the wall of the mesenteron, where their nuclei and out- 

 lines can easily be seen in entire specimens mounted either in 

 glycerine or in balsam. Every stage in this progressive migra- 

 tion may easily be seen, and as the cells pass inwards they dis- 

 appear correspondingly from the surface, though one or two 

 small pigment-spots remain until a rather late period. The 

 anus is finally formed in the middle of the area formerly occu- 

 pied by the pigment-cells. 



I have not yet fully investigated this process by means of 

 section, and hence must speak of it with some reserve. It is 

 possible to suppose that no actual inward migration takes 

 place, and that the appearance is caused by the successive de- 

 velopment of pigment in the inner cells and its concomitant 

 disappearance from the outer. All the appearances, however, 

 speak for an actual migration of the cells, and actual sections 

 of the earlier stages show the cells of the pigment-area much 

 elongated, often with the bodies of the cells lying below the 

 surface, with long pointed ends directed outwards. 



The longitudinal muscles {l.m., Fig. 85) are differentiated at 



