62 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



dimension of the cell. The blastomere d*-^ (Plate 2, Figs. 14 and IG) 

 is so exceedingly irregular in form, that it is impossible to determine 

 with certainty which is the longest axis. The cells a^-^-c*'^ are irregu- 

 larly triangular, and the direction in which the spindle lies is the short- 

 est line connecting any apex with the middle of the opposite side (Plate 

 3, Figs. 18, 19). Finally, the cells a*-^ - c*-^ are approximately rectan- 

 gular in form, with one of the axes much longer than the other ; the 

 spindles lie in the shorter axes (Figs. 20-22). In every case the spindle, 

 whatever the form of the cell, lies in a meridian connecting the animal 

 pole with the vegetative pole of the egg. The only rational conclusion 

 from this fact is, that the position of the spindles is determined by 

 some factor unconnected with the form of the cells. 



The theory that the direction of the spindles is due to their taking a 

 position of equilibrium determined by the mutual attraction of spindle 

 and protoplasm, so strongly insisted upon by Ziegler ('94, p. 140), is 

 likewise inconsistent with the movements of the asters in the eight-cell 

 stage of Asplanchna. Ziegler holds that, since the greater mass of pro- 

 toplasm must exercise the greater attraction, the spindle in the short 

 axis of the cell is in a position of unstable equilibrium ; if by any 

 cause it is moved in the slightest degree to one side or the other, it 

 must inevitably swing into the long axis of the cell, where alone it can 

 be in a position of stable equilibrium. This he holds to be the explana- 

 tion of the movements of asters and nuclei observed by Auerbach in 

 nematodes, and by himself in nematodes and echinoderms, as mentioned 

 above. An oblique position of asters and nuclei, such as is shown in 

 the cell a*-\ Figure 22 (Plate 3), is intelligible on this assumption if 

 the movement taking place is from the shorter toward the longer axis. 

 But in this cell, as in the other five of the quadrants A, B, and Cat 

 this stage, the movement is from the longer axis toward the shorter. 

 The hypothesis that the movement is due to simple attraction between 

 the protoplasm and the fundament of the spindle, varying with the »m£s 

 of the protoplasm, is totally inconsistent with such a notion. 



The passage from eight to sixteen cells is not the only cleavage in 

 Asplanchna which is irreconcilable with Hertwig's principle. At the 

 transition from the sixteen-cell to the thirty-two-cell stage, there is a 

 similar regularity in the position of the spindles coincident with variety 

 in the form of the cells. The four dorsal cells a^**- c?^-* divide equa- 

 torially, three of them with spindles in the longer axis; one, d^'*, with 

 the spindle in the shorter axis (Plate 4, Fig. 3.3). 



Again, in the sixth cleavage the cell d^-^ shows the same phenom- 



