No. 3.] 



THE CELL- LINEAGE OF NEREIS. 



427 



Diagram V. 



A. Early trochophore of Nereis from the left side; a^, d'^, a^, a^-^, d^-'^, t/^'i, 

 x'^, regions derived from the corresponding micromeres; f.li , frontal bodies; l.d., 

 latero-dorsal region; m.d., median dorsal region; ;«., mesoblast axis; n., neural 

 axis; n.p., cephalic neural plate; p.c, polar cells; p.a., pigment-area. The cephalic 

 neural plates, the pigment-area, and the mesoblast bands are stippled; the ventral 

 plate (product of X) shaded with lines. The dark body is the eye-spot. 



B. Corresponding view of later stage. Beginning of the shifting of the neural 

 axis. Concrescence of the ventral neural plates. 



C. Diagram of the three-segmented larva (adult condition). Completion of the 

 axial shifting, coincidence of the neural and mesoblast axes. Shifting of the frontal 

 bodies and of the ce]5halic neural plates. 



D. Young larva of Lopadorhynchus from the right side, before the shifting of 

 the axes. 



