No. 3.] 



THE CELL-LINEAGE OF NEREIS. 



405 



The first somatoblast {X) and its progeny {x^, x^). 



The third group of micromeres {a^, b^, ^, d^). 



The second somatoblast {M). 



The arrangement of these 9l. 



cells with respect to the 

 blastopore is shown in Dia- 

 gram III. The entomeres 

 are considerably more than 

 half enclosed by the other 

 cells (Figs. 33, 34), and the 

 open space may be called 

 the blastopore. It will be 

 seen from the diagram and 

 from a comparison of the 

 figures, that the blastopore 

 is somewhat asymmetrical, 

 and that its hinder lip 

 (which is formed by the 

 second proteloblast) lies 

 somewhat posterior to the 

 lower pole (which lies in 

 the middle of the cross- 

 furrow). The closure of the blastopore takes place precisely at 

 the lower pole (Figs. 55-57) by convergence of the cells from all 

 sides, though the shape and position of the blastopore are such 

 that the principal growth of the cells during the closure takes 

 place from in front backwards {i.e. as in Rhynchelmis or Clepsine, 

 and not as in Lumbric7is). 



The margin of the blastopore is formed at first by all of the 

 cells of the lower hemisphere excepting x^ [cf Diagram III and 

 Figs. 25, 26, 31, 33). As it narrows (Fig. 56), its anterior and 

 lateral margins are formed in part by the descendants of a^, b^, c^, 

 and d^, and in part by three large cells {st) zvhich give rise iji 

 large part at any rate to the stojnodceiim, and may, therefore, be 

 called stomatoblasts. The stomatoblasts, for reasons to be here- 

 after set forth, I believe to be the cells a^-'^, b'^-'^, and c^-"^, or their 

 derivatives. 



TJie posterior lip is foj-med by a group of small cells that are 

 budded forth from the primary mesoblasts derived by the bilat- 

 eral division of tJic first somatoblast (Figs. 56, 57). These cells 



Diagram III. 



