No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 263 



The &%% is oriented in precisely the same way as is AmpJii- 

 trite. Up to the time when the sperm enters the ^^g, the first 

 maturation spindle is in the equatorial-plate stage, and the 

 polar globules remain to mark the apical (animal) pole. The 

 first furrow depresses the surface first at the animal pole, as in 

 Nereis. 



A detailed account of the later cleavage processes in Cfytne- 

 nella would be little else than a repetition of the above de- 

 scription of Amphitrite. Every division up to the 6^-cell stage 

 takes place in the same direction in both species. There is, 

 moreover, a most striking resemblance in the relative size of 

 the cells, though the differences demand attention : the cell D^ 

 is larger in proportion than in Amphitrite ; the four upper cells 

 of the 8-cell stage, and therefore the whole anterior hemisphere 

 of Clymenella is relatively smaller — among themselves, how- 

 ever, the cells in this hemisphere have about the same relative 

 size as in Amphitrite ; the mesoderm cell M is comparatively 

 large; the sixteen descendants of the trochoblasts (i6-cell stage) 

 have the same arrangement, and all become prototrochal cells 

 in the same manner ; the apical rosette and cross arise in the 

 same way; the seven cells, which at the 64-ceIl stage constitute 

 the entoderm plate, are in the same position ; the relative size 

 of a^-% b'-% e-\ and x^'^ {d'-'-') is the same, x' {= d'-'-'), how- 

 ever, is somewhat smaller, and this difference may be of 

 considerable theoretical importance in connection with the 

 formation of the paratroch, since three-fourths of the paratroch 

 in Amphitrite is formed from this cell (PI. XV). 



Remarkable as are the similarities between the early cleav- 

 age stages in these two worms representing rather distantly 

 related families (Maldanidas and Terebellidae), some of the 

 subsequent phenomena show even more striking resemblances. 

 The apical rosette cells divide in the same manner (Figs. 71, 

 72). Each of the " secondary trochoblasts," a""'^, <^^'', and c"'^ 

 divides into a group of four cells : three larger and subequal, 

 and the fourth minute. These groups correspond in the three 

 quadrants. The three larger cells in each group become ciliated 

 and complete the prototroch, while the mijiute one does not enter 

 it. Thus, the whole prototroch in Clymenella arises in precisely 



