No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 24 1 



the corresponding cells in the other quadrants. They divide 

 somewhat in advance of the latter, and though d"^'' (= x") divides 

 in the same direction as the corresponding cells in the other 

 quadrants, i.e., continues in the alternating rhythm, ^'■'■' (= X^ 

 does not, but divides somewhat obliquely in the other directio7i 



(Fig. 38). 



The other somatic-plate cells, x^'^ and .r'"^ divide again ; ;r"'' 

 synchronously with the division just mentioned (Fig. 38), but 

 x"'^ considerably later (Figs. 41-44).* Both these divisions 

 are clearly contrary to the rule of alternating cleavage, for both 

 are in thej^w/^ direction as that of their parejit cell x^ (Fig. 20). 



A glance at Figs. 38-40 is sufficient to show that the somatic- 

 plate cells begin early to arrange themselves symmetrically 

 with regard to the middle line of the embryo: X^ is squarely 

 in the middle line, x' nearly so (see future divisions); x'^ on 

 the left side balances x^'^ on the right and divides symmetrically 

 with it. x^''', however, does not have a corresponding cell on 

 the other side. It is worthy of note that in many animals this 

 small cell arises in the same way, and has about the same pro- 

 portional size (Unio,^ Fig, 41, Nereis} Fig. 55, Clymenella, 

 Fig. 78, Chcetoptertis, Fig. 30). 



I have watched the cleavage of the somatic plate from two 

 principal points of view: that of developmental mechanics, and 

 that of the axial relationships, the shifting of areas, etc. 



The bilateral divisions of the somatic plate. — We have seen 

 that up to the 64-cell stage all the divisions took place strictly 

 in accordance with the rule of alternating oblique cleavage, 

 with no regard to bilaterality. But after this stage the cells of 

 the somatic plate divide withotit regard to the rule of alternating 

 cleavage, and with marked bilaterality . Of the two divisions 

 described last, that of x^ might possibly be considered as a con- 

 tinuation of the alternating cleavage (Fig. 38). The division is 

 oblique and to the right ; but since it has its mirrored image 

 in the division of x^'^, I believe, to state it paradoxically, that 

 x" divides obliquely to the right for the same reason that x^''' 

 does not — to conform to bilateral symmetry (Fig. 38, text 

 Fig. IV). 



* I have seen this cell in process of karyokinesis many times. 



