No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 245 



descended from x^, the remaining one {7'ight ventral) from x^ : 

 in other zuords, the matej'ial of three of the cells was separated 

 from that of the fourth, at the first division of the somatoblast 

 X {d"", l6-cell stage), and, as we have seen, has had a different 

 experience. (5) NotwitJistanding the differences in their past 

 history, the four cells are symmetrically located and are of the 

 same generation {I I th generation re^note from the ovtmi). 



To return to the somatic plate at the 64-cell stage — X^ divides 

 to form x^ and X^: X^, the large cell, divides into two equal 

 cells, lying one on either side the middle line; X^j. and X^^, 

 and their products divide bilaterally, and are symmetrically 

 placed, Figs. 43-55; X^ and X^ divide into X^ and X^ and x<^ 

 and x'^] X^ and X^ into X^ and X^, and x^ and x^ ; X^ and X^ 

 into X^ and Xg, and x^ and x"^. After this x^ divides very 

 unequally into x^'"' and x^"' ; then A^g and Xg divide into X, 

 and X^, and x^ and xfi. At about this time x^ and x^, xy and 

 xy, and x^'" all divide bilaterally and equally. 



The cleavage of the I'etnainifig ectoderm cells in the posterior 

 hemisphere. — We have already described the fate of the sec- 

 ondary trochoblasts, page 240, and showed that, with the 

 exception of three cells not much larger than polar globules, 

 all their descendants enter into the prototroch (cf. the latero- 

 dorsal trunk region in Nereis,'^ pp. 419, 427). 



The remaining ectodermal cells fall naturally into groups; 

 those descended from «°'^ U'''^, c", and from a^, b^, c^, and d^ of 

 the 32-cell stage, — we have already described the division of 

 each of these cells in the left oblique direction. There is little 

 to be gained from a detailed description of the further cleavage 

 of a^''^, b'^'^, and c'^'^, though I followed it in the eggs in order 

 (i) to be sure of the boundaries of the groups, (2) to become 

 acquainted with any landmarks which might develop, (3) to see 

 whether in these late stages there is the same constancy in 

 the direction of division and in the size of the blastomeres 

 in different individuals, that obtains in the earlier stages, and 

 (4) to obtain data for close comparison with other species, (i) 

 In Fig. 52 the boundaries of these groups are indicated. 

 (2) The rows of small cells with deeply staining nuclei, not far 

 from the edge of the somatic plate, are important landmarks 



