No. 2.] ' CLEAVAGE OF ARENICOLA CRISTATA. 



73 



of the first quartet of ectomeres have divided several times, 

 and sixteen primary trochoblasts have been formed (Fig. 5, tr\\ 

 Fig. 6, tr I). Various divisions have occurred in other blasto- 

 meres except the mesoblast (Fig. 5, J7). Figure 6 shows the 

 passage from the 58-cell stage to the next from the upper pole. 

 The four spindles here represent the first bilaterally symmetrical 

 division and also the formation of the apical cross of eight cells. 

 The second bilaterally symmetrical division in the egg occurs in 

 the largest derivative of X at the /o-cell stage. It is followed 

 immediately by the symmetrical division of M into right and 

 left halves. At a stage when the egg consists of over a 

 hundred cells, a bilaterally symmetrical division occurs in the 

 entomeres, the last division that they undergo before the 

 blastopore closes. It is an interesting fact that in each of 

 the cases mentioned, viz., the cells of the first quartet, the 

 derivative of X, the mesoblast and the entomeres, this first 

 bilaterally symmetrical division occurs in cells of the same gen- 

 eration, the eighth, counting the unsegmented egg as the first. 

 The later derivatives of the cells just mentioned all divide 

 symmetrically, though in derivatives of the first quartet, I have 

 often seen what is apparently a partial return to the oblique 

 type in the direction of the spindle. Other cells of the egg 



FIG. 3. 



FIG. 4. 



continue to divide obliquely up to a stage shortly before the 

 closure of the blastopore, when, I think, all blastomeres are 

 dividing symmetrically or approximately so. The sixteen 

 primary trochoblasts do not divide further, but do not become 

 ciliated until some time after the blastopore closes. 



