72 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
groups itself symmetrically about it, until just before division the 
spindle lies in the shorter axis of the cell, and a plane including the 
long axis of the spindle and the centre of the egg would divide the cell 
into symmetrical halves. 
If this be the true explanation, then the elongation of the cells in 
Plate 10, Fig. 83, in various directions, is due to the previous altera- 
tion of the predetermined spindle axes. And certainly the elongation 
is not greater than is naturally the result of the position of the spindle, 
as has been shown for other cells. 
This view is strengthened by the fact that in certain cases, where 
apparently it is impossible for the cytoplasm to group itself symmetri- 
cally about the spindle, the latter takes a position which is oblique to the 
axes of the cell. Such a case is shown in the entodermic cell d™! of 
Figure 64 (Plate 8); the spindle lies in neither the longest nor the 
shortest axis of the cell, but oblique to both. 
It must be said, therefore, that the cleavage of Asplanchna gives no 
positive evidence of any effect of the form of the cell upon the position 
of the spindle ; on the contrary, the evidence on the whole is decidedly 
against it. This, of course, cannot be generalized and applied to other 
organisms, as on the other hand the observed conditions in other organ- 
isms are not capable of giving generalizations which must hold for 
Asplanchna. Generalizations in the field of reaction to stimuli, which 
includes a very large proportion of organic activities, are exceedingly 
unsafe and are justifiable only after exhaustive examination of the phe- 
nomena; but in these matters scarcely more than a beginning of such 
an examination has been made. 
(5) Heidenhain’s problem of a definite angle of rotation. (Compare 
page 6.) — This certainly cannot be held to have any especial signifi- 
cance for the cleavage of Asplanchna ; it is proposed by Heidenhain as 
applicable to systems of tissues in the later stages of organisms. I may 
point out some facts which bear upon the “ problem.” 
In the cleavage of Asplanchna, the angle which a given spindle makes 
with the preceding spindle may be either 0 or 90 degrees ; but it is gen- 
erally either one or the other, not some angle intermediate between 
these. 
The time and method of rotation, when rotation occurs, cannot be 
said to be “ gesetzmässig.” Commonly the asters come to lie on oppo- 
site sides of the nucleus before the rotation begins, but in the cells d&1 
(Fig. 42, Plate 5) and d*? (Fig. 37) the change of position begins at 
the same time with the division of the asters, so that by a separation of 
