IV] AND STRUCTURE OF THE CELL 311 



penneability is greater or less than that of the surrounding medium'^. 

 In the common experiment of placing iron-filings between the two 

 poles of a magnetic field, the filings have a very high permeability; 

 and not only do they themselves become polarised so as to attract 

 one another, but they tend to be attracted from the weaker to the 

 stronger parts of the field, and as we have seen, they would soon 

 gather together around the nearest pole were it not for friction 

 or some other resistance. But if we "repeat the same experiment 

 with such a metal as bismuth, which is very little permeable to the 

 magnetic force, then the conditions are reversed, and the particles, 

 being repelled from the stronger to the weaker parts of the field, 

 tend to take up their position as far from the poles as possible. 

 The particles have become polarised, but in a sense opposite to that 

 of the surrounding, or adjacent, field. 



Now, in the field of force whose opposite poles are marked by 

 the centrosomes, we may imagine the nucleus to act as a more or 

 less permeable body, as a body more permeable than the surrounding 

 medium, that is to say the " cytoplasm " of the cell. It is accordingly 

 attracted by, and drawn into, the field of force, and tries, as it 

 were, to set itself between the poles and as far as possible from both 

 of them. In other words,' the centrosome-foci will be apparently 

 drawn over its surface, until the nucleus as a whole is involved 

 within the field of force which is visibly marked out by the "spindle" 

 (Fig. 90 b). 



If the field of force be electrical, or act in a fashion analogous 

 to an electrical field, the charged nucleus will have its surface- 

 tensions diminished f: with the double result that the inner alveolar 

 mesh work will be broken up (par. 1), and that the spherical 

 boundary of tKe whole nucleus will disappear (par. 2). The break- 

 up of the alveoli (by thinning and rupture of their partition walls) 



* If the word penneability be deemed too directly suggestive of the phenomena 

 of magnetism, we may replace it by the more general term of specific iyidiictive 

 capacity. This would cover the particular case, which is by no means an improbable 

 one, of our phenomena being due to a "surface charge" borne by the nucleus 

 itself and also by the chromosomes: this surface charge being in turn the result 

 of a difference in inductive capacity between the body or particle and its surrounding 

 medium. 



t On the effect of electrical influences in altering the surface-tensions of the 

 colloid particles, see Bredig, Anorganische Fermente, pp. 15, 16, 1901. 



