No. 3.] 



THE CELL-LINEAGE OF NEREIS. 



415 



is invariably found a larger cell (Figs. 55 to 57), that I for a long 

 time mistook for a product of the primary mesoblast, but which 

 is unquestionably d'^ (the left posterior micromere of the third 

 group or one of its progeny), as will be shown later. Corre- 

 sponding to it on the other side, adjacent to the right stomato- 

 blast, is c^ (the right posterior micromere of the third group), 

 and between this and the secondary mesoblast-cells is a cell 

 which I believe to be one of the progeny of x^. It will be 



V 



T. sIt 



V 



h 



st.vw. 



i 



C 



A 



a 





•stl. 1. 



1^ 



Diagram IV. 



observed that the margin of the blastopore is not quite sym- 

 metrical, since it includes one of the ;ir-cells on the right side. 

 This curious asymmetry is, I believe, quite constant, and ap- 

 pears to be caused simply by the displacement of the second 

 somatoblast towards the left, so as to come into juxtaposition 

 with <^^ and st.l. (Fig. 53), and thus separate ,t-^ from the blas- 

 topore. It is by the convergence of this circle of cells that 

 the blastopore closes ; and, before taking up the rather compli- 

 cated phenomena involved in the closure, I will endeavor to 

 trace the origin of the cells in the cleavage. The condition of 

 the blastopore at the end of the spiral period has been described 



