spindle fibers are real and capable of expansion and contraction, and 

 in so doing push and pull the chromosomes into position. There is little 

 positive evidence for this view, and in some cases changes in the spindle 

 are directly contrary to expectation. 



Electrostatic Forces 

 The idea that the chromosomes may move as a result of the distribu- 

 tion of electric charge was originally advanced by R. S. Lillie (1905). 

 This suggestion has had considerable appeal, but becomes extremely 

 complex when applied to meiosis and almost hopelessly unwieldy in a 

 variety of exceptional cases. That there are areas in the cell bearing 

 different charges is almost certain, and it seems more than probable 

 that electrostatic forces do play some role in chromosome movement. 



Hydrodynamics 

 The physicist Bjerknes (1902, 1909) made an extensive study of the 

 relationship between pulsating bodies, oscillating bodies, and bodies in 

 their vicinity which were static. He found that ( 1 ) two spheres pulsating 

 in phase attract each other and in opposite phase repel; (2) such 

 spheres set up lines of force not unlike a spindle in appearance; (3) a 

 neutral sphere is repelled by a pulsating one if it is lighter than the 

 medium, but is attracted if heavier than the medium; (4) oscillating 

 spheres behave in the same fashion except that they repel when in the 

 same phase and attract when in opposite phase. With these facts it is 

 possible to build up a superficially satisfactory hypothesis of mitosis, but 

 it is difficult to see how the metaphase plate is formed, and it is also 

 unlikely that the forces concerned would act over the necessary distances. 



Tactolds 

 The fact that long fibers suspended in an unoriented medium will line 

 up in a spindle-like form is well known. The major attempts to apply 

 this fact to mitosis have been made by Bernal (1940), Ostergren 

 (1945, and later), and Schrader (1951). Too little is known of tactoids 

 from the physiochemical point of view to allow any reasonable assess- 

 ment of their potential role in mitosis. 



Other Forces 

 Such forces as electronic diffusion, sol-gel transformation, and cyto- 

 plasmic streaming have been suggested, but though they may play a part, 

 none of them seems capable of giving the organization observed. Like- 

 wise autonomous chromosome movement has been stressed as a factor, 



144 / CHAPTER 6 



