No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 239 



cells {gl., text Figs. XI-XVII), it seems extremely probable 

 that the middle cells in the ventral, as well as in the dorsal arms 

 of the cross, give rise to unicellular mucous glands. The out- 

 lines of the cross are destroyed by the further cleavage of its 

 component cells (Figs. 36, n). These later divisions are 

 interesting because they are manifestly bilateral, and because 

 they bear a remarkable correspondence to the same divisions 

 in Nereis, as far as the latter are figured (Nereis} Figs. 41-44). 



Of the intermediate cells only those between the dorsal arms 

 of the cross have been followed through several generations. 

 My object in following these was to ascertain what cells, if 

 any, migrate from the anterior to the posterior hemisphere, 

 through the mid-dorsal interruption of the prototroch, that is, 

 through the gap between the prototrochal cells of the d and c 

 quadrants. In the 64-cell stage these cells are two in number, 

 ^'■^■' and ^''^'^ (Figs. 18, 24, 29). The latter is much larger and 

 divides first, obliquely to the right (Fig. 30), and the former 

 divides in the same direction (Fig. 45, d^'^''^).* The posterior 

 product of d^'^''^ divides and for the three descendants of d^''"'' 

 we will substitute the letters, /.', /.^ and U (Figs. 32, 34-36, 45). 

 Next 1} divides (Fig. 51) and then /.^ making five cells in the 

 group (Fig. 54). By this time the interruption in the proto- 

 troch has become much narrowed by the approach of the huge 

 prototrochal cells from either side, and the /. group is seen to 

 have taken a position below the narroivest part (Figs. 58, 60). 

 Meanwhile the anterior product of <^'"" divides. One of the 

 daughter cells is extremely minute, has a deeply staining 

 nucleus, and serves as an excellent landmark (asterisk. Figs. 

 3Sj 36, 51, 54, 58, 59, 60). When the prototrochal cells have 

 met in the middle line, this minute cell with its twin, which 

 also has a peculiar appearance, lies just above the suture (Fig. 

 60). 



Therefore, returning to Fig. 24 or 29, I think we are 

 warranted in saying that about half the cell d^''^'' and possibly 

 part of d'"^''^ contribute to the structures posterior to the pro- 



* In the figures, the progeny of </'-^'' are indicated by the heavy outlines of 

 their nuclei, while the posterior product of the division of if' -^-^ (and its descend- 

 ants) are distinguished by shaded nuclei (Figs. 34-36). 



