62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
dimension of the cell. The blastomere d*? (Plate 2, Figs. 14 and 16) 
is so excecdingly 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*t- 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 
aud protoplasm, so strongly insisted upon by Ziegler (794, p. 140), is 
likewise inconsistent with the movements of the asters in the cight-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 coll att, 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 4, B, and C at 
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 mass 
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 Hertwie'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 coineident with variety 
in the form of the cells, The four dorsal cells, a*t- d divide equa- 
torially, three of them with spindles in the longer axis; one, %, with 
the spindle in the shorter axis (Plate 4, Fig. 33). 
iu, in the sixth cleavage the cell d^? shows the same phenom- 
