THE MECHANISM OF MITOSIS 75 



tension of the astral rays which are attached at one end to the cen- 

 trosome, at the other to the cell-periphery. By turgor of the cell 

 the rays are passively stretched, thus causing divergence of the 

 spindle-poles and of the daughter-chromosomes to which the spin- 

 dle-fibres are attached. An active contraction of the fibres is only 

 invoked to explain the closing phases of mitosis. 



(U) Other Theories. Watase's ingenious theory of mitosis ('93) 

 is exactly the opposite of Van Beneden's, assuming that the spindle- 

 fibres are not pulling but pushing agents, the daughter-chromo- 

 somes being forced apart by continually lengthening fibres which 

 grow out from the centres and dovetail in the region of the inter- 

 zonal fibres. Each daughter-chromosome is therefore connected 

 with fibres from the aster, not of its own, but of the opposite 

 side. This view is, I believe, irreconcilable with the movements 

 of chromosomes observed in multiple asters, and also with those 

 that occur during the fertilization of the egg, where the chromo- 

 somes are plainly drawn towards the astral centres and not pushed 

 away from them. 



Butschli, Carnoy, Platner, and others have sought an explanation 

 in a totally different direction from any of the foregoing, regarding 

 the formation of the amphiaster as due essentially to streaming or 

 osmotic movements of the fluid constituents of the protoplasm, and 

 the movements of the chromosomes as being in a measure mechan- 

 ically caused by the same agency. Oscar Hertwig adopts a some- 

 what vague dynamical view, regarding the formation of the mitotic 

 figure as due to an interaction between nucleus and cytoplasm, which 

 he compares to that taking place in a magnetic field between a mag- 

 net and a mass of iron filings : " The interaction between nucleus and 

 protoplasm in the cell finds its visible expression in the formation of 

 the polar centres and astral figures ; the result of the interaction is 

 that the nucleus always seeks the middle of its sphere of action." 1 

 He gives, however, no hint of his view regarding the nature of the 

 action or the causes of the chromosomal movements. Ziegler ('95) 

 accepts a somewhat similar view ; and he has shown that surpris- 

 ingly close simulacra of the mitotic figure in many of its different 

 phases may be produced by placing bent wires (representing the 

 chromosomes) in the field of a horseshoe magnet strewn with iron 

 filings. 



My own studies on the eggs of echinoderms ('95, 2) and annelids 

 have convinced me that no adequate hypothesis of the mitotic mech- 

 anism has yet been advanced. In these, as in many other forms, the 

 spindle-fibres show no differentiation into central spindle and peri- 



1 Zelle und Gewebe, p. 172. 



