98 BULLETIX: MUSEUM OF COMPARATIVE ZOOLOGY, 



being the same as in the other three quadrants (see Figs. 33 and 37). 

 In Asplanchna, moreover, the fifth cleavage in the other quadrants is 

 well advanced before any of the divisions for the sixth cleavage in 

 quadrant D have taken place. 



Zelinka does not follow the cleavage in the other three quadrants cell 

 by cell. He states that the dorsal cells of these quadrants (all T) are 

 divided by planes parallel to the long axis of the egg, as is the case for 

 all but the fourth or most dorsal layer in Asplanchna, and that the 

 ventral cells a^"\ U"'^, aud c""-^ (rti, h-^, and II, Zelinka) are the last to 

 divide. In Asijhuichua, as shown in Figures 39-44 (Plates 5 and 6j, 

 all the cells of tliese quadrants divide meridionally except those of the 

 dorsal layer, which divide equatorially. 



It is obviously impossible to compare in detail the cleavage in the two 

 forms at this time, or to reduce the condition described for Callidina to 

 the regular scheme of cleavage exhibited in Asplanchna. Certain facts 

 are perhaps worthy of notice, as showing the possibility that the cleavage 

 in Callidina is not so different from that of Asplanchna as would be 

 inferred from what is indicated above. The relation of the cells in 

 quadrant D are somewhat similar in Figures 30 and 34 of Zelinka's 

 work to a later condition in Asplanchna, — a condition reached, how- 

 ever, in a very different way, aud shown in Figures .58 (Plate 7) and 66 

 (Plate 8). Further, the unequal cleavages of this quadrant are very 

 confusing, and easily overlooked. Zelinka's work was apparently done 

 almost entirely on living material, which does not lend itself as well as 

 does i)reserved material to precise orientation of the object, and to its 

 rotation in such a way as to permit views from all directions. It was 

 only by bringiiig together a complete series, in which the karyokinetic 

 process in every cell was represented in various stages by several 

 specimens, that I was able to determine absolutely the course of events 

 here. The remarkable unequal division of the cell d^-^ is especially 

 liable to be overlooked ; I did not observe it till the break in the rhythm 

 of cleavage at this point set me at work upon a minute study of a series 

 of eggs separated in cleavage conditions by very short intervals only. 

 It is worth noting that in Callidina, shortly after the time for this 

 cleavage to occur, Zelinka figures (Taf. II. Fig. 31) a small vesicle lying 

 almost exactly in the place occupied by the minute vesicle given off at 

 this cleavage in Asplanchna, viz. between the ventral cells of quadrants 

 A and B {a^ and 5i, Zelinka). Zelinka considers this to be the polar 

 cell, although at a previous stage he had observed that the polar cell 

 had become disj)laced and now lay farther dorsad, on the outer surface 



