No. 3.] 



THE CELL-LINEAGE OF NEREIS. 



463 



LETTERING OF THE PLATES. 



The blastomeres are lettered according to the following system, for an illustration 

 of which see Diagram, p. 378: — 



A, B, C, D. The four primary blastomeres and the corresponding macromeres or 

 entomeres. 



d\, b^, c^, if^. First group of micromeres. 



a"^, 32, ^2^ d^ (X). Second group of micromeres. 



a^, b^, <^, d^. Third group of micromeres. 



Their products are in general lettered according to the principle illustrated in 



