160 INTRODUCTION TO CYTOLOGY 



trate the supposed contraction and its results (Heidenhain) ; but evidence 

 subsequently brought forward ^^ led to the general restriction of the role 

 of contractility, until it became apparent that this factor must be one of 

 minor importance. 



The striking resemblance between the achromatic figure and the lines 

 of force in an electromagnetic field naturally led to attempts to account 

 for mitosis on the basis of electrical principles. Several investigators, 

 working with various chemical substances, succeeded in modeling fields of 

 force that illustrated graphically the changes supposed to take place in 

 mitosis. In later years the electromagnetic interpretation was again 

 brought into prominence by Gallardo, Hartog, and Prenant. At first 

 Gallardo (1896) believed the two spindle poles to be of unlike sign, but 

 later (1906), as the result of the researches of R. S. Lillie (1903) on the 

 behavior of nucleus and cytoplasm in the electromagnetic field, he con- 

 cluded that they are of like sign, the centrosomes moving apart because of 

 their similar charge. The movement of the chromosomes to the poles he 

 held to be due to the combined action to two forces: a mutual repulsion of 

 the similarly charged daughter chromosomes and an attraction between 

 the oppositely charged centrosome and chromosome. It has also been 

 thought that the chromosomes may assume their equatorial position 

 because they are repelled by both poles, and that a later weakening of 

 the repelling force relative to the mutual repulsion of the chromosome 

 halves allows the latter to move poleward. 



The fact that the two centrosomes and hence the two spindle poles are 

 electrically homopolar (Lillie) at once made it apparent that the mitotic 

 figure does not represent an ordinary electromagnetic field, for in the 

 latter the poles are of unlike sign — the field is heteropolar. It was conse- 

 quently suggested by Prenant (1910) and Hartog (1905, 1914) that the 

 mitotic figure is the seat of a special force peculiar to living organisms. 

 This force they called "mitokinetism. " It can scarcely be doubted that 

 electrical forces are in some measure concerned in the mitotic changes, but 

 it is impossible at present to state how they act.^^ Any theory involving 

 such forces must be applicable not only to ordinary bipolar mitosis but 

 to occasional tripolar, quadripolar, and unipolar (p. 163) mitotic figures as 

 well. 



Special significance has been attached to streaming by students of 



mitosis ever since Biitschli, Fol, and others showed many years ago that 



currents usually exist in the protoplast. That streams passing poleward 



within the spindle play a role in chromosome movement was suggested by 



many of the aspects observed, and the subsequent discovery that the 



astral rays are such streams lent further plausibility to the view\ That 



the "spindle fibers" are such streams was, however, not proved. 



'2 Hermann (1891), Druner (1894, 1895), Calkins (1898), and others. 

 i^See WUson (1925, pp. 184-189). 



