72 BULLETIX: MUSEUM OF COMPARATIVE ZOOLOGY. 



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 sjmmetri- 

 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. TTiis, 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 cleaA-age 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 or 90 degi-ees ; 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 " gesetzmassig." Commonly the asters come to lie on oppo- 

 site sides of the nucleus before the rotation begins, but in the cells d^'^ 

 (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 



