182 TORREY 



siderably in size. X^ and X^ next divide at about the same time, 

 unequally and dexiotropically (PL II, i8 and 19). The smaller 

 product lies nearer the prototroch in each case. It is a signifi- 

 cant fact that tlie divisions in this qiindrant have considei^ably out- 

 stripped those in the other quadrants, but are almost identical 

 with the ones that ivill occur in them later. This is the first in- 

 dication that the subsequent posterior growth will be due to the 

 multiplication and increase in size of the cells of this region. 

 The radial symmetry of the earlier stages, on the other hand, 

 persists very tenaciously in this form as in all annelids with equal 

 cleavage. The cell X, o enlarges somewhat (PL II, Fig. 20), 

 but not nearly so much as in Podarke. The group now has 

 assumed the shape of an open fan and retains it to a late 

 period (Text-Fig. 3, 6" and Z^). The cells A'"^ 2 ^^id A"., o divide 

 at the same time dexiotropically (PL II, Figs. 20 and 21). 

 In the division q>{ X ^ ^ ., c, the spindle lies toward A'g o (PL 

 II, Fig. 20, shows result of division). The upper prod- 

 uct of this division, X^ j 9. 2. 1 later increases considerably in 

 size and becomes by far the largest cell in the group (PL II, 

 Fig. 24). X ^ 1 o. 1 buds off toward the prototroch a very small 

 cell (A^i.i. 2. 1. 1, PL II, Figs. 20 and 21) which very soon sinks 

 into the cleavage-cavity. By this time gastrulation has begun, 

 accordingly the subsequent fate of these cells will be described 

 in a following section. 



a, b and c quadrants. — The cleavages in the other three 

 quadrants may be passed over very briefly. The small cells 

 budded off toward the prototroch (2^2. i> ^tc, PL II, Fig. 15), I 

 have never seen divide again, and I am sure that they later de- 

 generate and sink into the cleavage-cavity. Because of the 

 peculiar shingle-like arrangement of the post-trochal blasto- 

 meres, the cells budded off upwards from the second quartet lie 

 almost entirely under the primary prototroch (PL II, Fig. 13 ; 

 Text-Fig. 10, A). In the later stages, however, in conse- 

 quence of the thinning and stretching out of the body wall, this 

 condition to a large extent disappears (Text-Fig. 10, C^. The 

 dextral cell in each of these quadrants [2a^, etc.) now divides 

 nearly equally and leiotropically (PL II, Figs. 16 and 17). 



