No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 24^ 



The succeeding bilateral divisions of x'''^ and ;tr''° are of 

 special interest, for, although the cells, as a group, are not 

 placed quite bilaterally with respect to the embryo, their next 

 division corresponds in direction (Figs. 41, 42 and text Fig. 

 IV, a, b). The products of x"'"" are subequal, and of about 

 the same size as the adjacent larger product of x"'^-^, the latter 

 having divided unequally. This division gives the impression 

 of an effort towards bilateral cleavage in cells not perfectly bal- 

 anced with respect to the middle line. As a result, the three 

 subequal cells lie in a transverse row, one in the median plane. 

 The small cell on the extreme right apparently has little to do 

 with the balance of symmetry: it has no fellow on the left 

 side (Figs. 41, 42 and text Fig. IV, b). 



The middle cell of the group divides as an unpaired median 

 cell usually does in bilateral cleavage, i.e., me>idionally and 

 equally ; the cells on either side of it divide bilaterally — the 

 division of either being the mirrored image of that of the other. 

 As a result, we have six cells arranged in a bilaterally sym- 

 metrical group, whose plane of symmetry falls slightly to one 

 side of the middle line of the embryo (Figs. 43-47, text Fig. IV). 

 This being the case, it is obvious that, if the next division 

 should take place sy^nmetrically with reference to the 7niddle 

 furrow of the group, the arrangement of the resulting cells would 

 be asymm-etrical with reference to the middle line of the embryo. 

 And conversely, if, after the next division, the cells should be 

 arranged symmetrically with regard to the middle line of the 

 embryo, they must have divided with total disregard of the 

 symmetry of the group. 



Which course will the cleavage pursue ^ As compared with 

 the bilateral cleavage in eggs like the squid and ascidians, where 

 the plane of symmetry of a group of cells coincides with that 

 of the embryo, and where the origin of the cells on one side is 

 the same as that of the corresponding cells on the other, the 

 case we have in hand is very complex. — Here we have a 

 bilaterally symmetrical group of cells somewhat asymmetrically 

 placed, and the origin of the cells on one side is different from 

 that of the corresponding cells on the other; they do not even 

 belong to the same cell generation. To my mind this is a 



