No. 3.] THE CELL-LINEAGE OF NEREIS. 389 



ectoblast of the lower hemisphere. The large posterior min'o- 

 mere or first somatoblast gives rise to the entire ventral plate of 

 the larva, and hence to its products, the ventral nerve-cord, the 

 seta-sacs, and probably a portion of the nephridia. 



The position of these cells should be clearly understood {cf. 

 Figs. 14 and 17). The lower hemisphere is occupied by the 

 four macromeres, A, B, C, D, which have the same arrangement 

 as in the four-celled stage, the lower cross-furrow remaining quite 

 unchanged. The nucleus of each lies in its upper portion (Fig. 

 17), towards the left side, as seen in side view. Around the 

 upper pole lie the four primary micromeres {a}-, b^, c^, d^) with 

 the four trochoblasts adjoining them. Of the three smaller 

 micromeres of the second group {d^, b'^, c^), b"^ lies in the median 

 line in front, opposite the cleavage-line between A and B, while 

 a^ and c^ lie symmetrically, one on either side, a^ opposite the 

 cleavage-line between A and D, c^ opposite the corresponding 

 line between B and C. The first somatoblast, X, which has 

 assumed an oval form, occupies the posterior region of the 

 embryo, opposite the cleavage-line between (7 and D, but some- 

 what to the left of tJie middle line. This slightly asymmetrical 

 position of X is not only perfectly constant in this stage, but it 

 may be traced for a long time in the progeny of X. Ultimately, 

 however, it is rectified, and the numerous descendants of X are 

 bilaterally arranged with respect to the median line of the body. 



It is clear, however, that in the sixteen-celled stage the embiyo 

 is not bilaterally syimnetrical. The arrangement of the cells is 

 radial, but in such wise that the radii do not pass straight out- 

 ward from the principal axis, but are bent to one side. This 

 arrangement, which may be termed a spiral symmetry, is re- 

 tained up to a late stage. 



Extremely clear optical sections are afforded by the acetic- 

 glycerine specimens after staining with Schneider's carmine. 

 They show (Fig. 18) that there is no trace of a segmentation- 

 cavity, all the cells being accurately fitted together. The 

 protoplasm, in such preparation, shows marked differentiations. 

 That of the primary and secondary micromeres and of the tro- 

 choblasts is granular and reddish in color. In the somatoblast 

 the granulation of the protoplasm is coarser and the color some- 

 what deeper, so that there is a marked contrast between it and 

 the others. In unstained specimens this contrast is still greater, 



