No. 3.] THE CELL-LINEAGE OF NEREIS. 401 



upper hemisphere contains a beautiful and symmetrical cross of 

 cells, the four arms of which, inclined at an angle of 45° to the 

 median plane, are the products of the four primary micromeres, 

 «\ b^, c^, d'^ respectively. The centre of the cross is occupied by 

 the apical rosette, and each arm consists of three cells, the ter- 

 minal of which is larger than the others, like a teloblast, and 

 may still be regarded as the parent micromere. For the sake of 

 brevity, I shall henceforward call them the pole-cells of the cross. 

 The middle cell of each posterior arm is the tiephroblast (hence- 

 forward designated as ;/). 



The pole-cells still lie in contact with the prototroch ; the 

 interspaces between the arms of the cross and the prototroch 

 are occupied by the descendants of the intermediate girdle- 

 cells, which I have not succeeded in following in detail beyond 

 the stage shown in Fig. 41. 



The later history of the cross may be briefly treated. Both 

 the pole-cells and their derivation continue to divide bilaterally 

 as long as the individual cells can be followed ; but, owing to 

 the appearance of longitudinal divisions {i.e. parallel to the arms 

 of the cross) in the cross-cells {cf. Figs. 40, 41), and to divisions 

 in the descendants of the intermediate girdle-cells, the outlines 

 of the cross become less distinct and finally quite indistinguish- 

 able. The pole-cells likewise ultimately disappear ; the anterior 

 pair are lost first, while the posterior pair persist, and continue 

 their symmetrical divisions up to a late period (Fig. 59). 



The second bilateral division of «^, b^ is accompanied by an 

 oblique division of c^-'', d'^-^ (Fig. 39). This is followed by the 

 third bilateral division of c^, d^ (giving rise to c^-'°, d^-^, Fig. 40), 

 which is nearly longitudinal (with respect to the arms of the 

 cross). At the same time, a^-'^ and ^^-^ divide longitudinally 

 (Fig. 40). Next, «^-^and b^-^ divide in the same plane (Fig. 41), 

 and a little later the anterior pole-cells divide (Fig. 41) in an 

 approximately transverse plane. 



At this stage each of the original primary micromeres has 

 undergone three spiral and three bilateral cleavages, in the last 

 four of which the posterior pair {c''-, d^) invariably preceded the 

 anterior pair. Beyond this point I have not been able to follow 

 the anterior pair [a^, b^), since they become lost amongst the 

 surrounding cells. The fourth bilateral cleavage of c^, d^ (giving 

 rise to c^-"^, d'^'', Figs. 42, 43, 44) is nearly at right angles to 



