538 BULLETIN OF THE 



this same substance toward the astral centres. There is nothing in the 

 appearance of the asters to warrant the conclusion that they at first 

 exercise a repulsive, and subsequently an attractive influence on the 

 same substance. Besides, the accumulation of the nuclear substance 

 along other portions of the spindle fibres than the equator seems most 

 naturally explainable as resulting from an attractive influence exerted 

 by the vitelline filaments, which, however, are presumably of the same 

 nature as the central mass of the aster, and ought therefore to operate 

 in the same manner on nuclear substance. The view that the equatorial 

 accumulation might be due to the mutual attraction of the elements 

 composing the nuclear disk, does not help to explain cases where (Limax, 

 Figs. 86-89) the segmentation spindle lies far to one side of the pro- 

 nuclei. Any scheme which admits that the substance forming the equa- 

 torial thickenings remains unaltered as regards its attraction or repulsion 

 for other constituents of the egg, encounters the fact that this substance 

 moves during successive periods in practically opposite directions. If 

 there were any means of making it probable that a change in the (elec- 

 tric or other) conditions of the nuclear substance itself takes place while 

 it tarries in the equator of the spindle, its movements might then be 

 explainable as the result of the uninterrupted action of a single polar 

 force, without involving the necessity of an entire reversal in the opera- 

 tion of the hypothetical influence. But for the present I see no way of 

 accounting for a change in the nature of the moving substance which is auy 

 more satisfactory than the assumption of a reversal of the moving /orcf. 

 There is one feature in the migration of the lateral halves of the 

 equatorial thickenings which has, I believe, never been called in question 

 by any of the observers who have studied objects in which the migra- 

 tion could be readily observed in the living cell. The separation is at 

 first rapid, but subsequently the rate of the movement diminishes. The 

 bearing of this fact on the location of the forces which induce the sepa- 

 ration has not, so far as I recall, been stated by any one. It has more 

 generally been held that the separation is due to a traction emanating 

 from the centres of the asters, which draws asunder the halves of each 

 of these thickenings. Others have assumed that it was the result of the 

 mutual repulsion of the halves. So far as the rate of the separation is 

 concerned, it appears to me to support the latter view ; for, assuming, as 

 is most natural, that there is no rapid change in the intensity with 

 which the force acts, a diminution in the efl'ect is what must necessarily 

 ensue with a constantly increasing distance between the source of the 

 force and the object moved. A gradually accelerated motion must, on 



