No. 3.] THE CELL-LINEAGE OF NEREIS. 395 



{c) In the third place, soon after the events just described, the 

 second somatoblast or mesomere divides into equal parts {M.M., 

 Figs. 32, 48, 49, 50). These are the primary meso blasts, v^hich 

 give rise to the mesoblast-bands. 



The embryo now consists of forty-two cells. Though a dis- 

 tinctly bilateral arrangement is now apparent, the peculiar dis- 

 placement of the somatoblasts towards the left is still apparent, 

 and it is only gradually overcome as the development pro- 

 gresses. 



2. History of the First Grojip of Micromeres. Origin of the 

 Cross and of the Prototroch. 



It will be useful at this point to recapitulate the history of 

 the girdle, since it is immediately connected with the origin of 

 the prototroch. The four primary micromeres first give rise 

 (at the fourth cleavage) to four smaller cells, the trochoblasts 

 {ct}--^, b^-^, c^-^, (P--^) in a right-handed spiral division (Figs. 13, 14), 

 and, then (at the fifth cleavage), in a left-handed spiral division, 

 to four similar cells alternating with the trochoblasts, which 

 may be called the intermediate girdle-cells {a}-'^, b^^, c^-^, d^-^). 

 Thus a girdle of eight cells is formed that surrounds the cen- 

 tral micromeres (Fig. 20). [In the figures the trochoblasts are 

 colored a deeper blue than the others.] Later, in the course of 

 the fifth cleavage, the trochoblasts divide in a meridional plane 

 (Fig. 23), so that the number of cells in the girdle increases from 

 eight to twelve. 



When fully formed the prototroch is likewise found to consist 

 of twelve cells, and I at first supposed that they were identi- 

 cal with the twelve girdle-cells. Further investigation proved, 

 however, that the four intermediate girdle-cells are drawn out of 

 the girdle, and the prototrochal cells are formed fivm the eight 

 remaining cells, derived from the trochoblasts. 



In the eight-celled girdle (Fig. 20) the cells are very regularly 

 arranged, two girdle-cells lying opposite each micromere. After 

 the division of" the trochoblasts, however, the intermediate 

 girdle-cells are gradually displaced, so that they first come to 

 lie opposite the intervals between the micromeres, and then 

 begin to extend up between them {cf. Figs. 23 to 30, and Dia- 

 gram II). The posterior median girdle-cell is the first to be 

 drawn out of the girdle, being pushed up between the products 



