ivl STRUCTURE OF THE CELL 177 



the neighbourhood of the centrosome of the ripe cell or ovum, 

 when it is about to divide : and by the time the centrosome has 

 divided, the field is definitely a bipolar one. 



The quality of a medium filling the field of force may be uniform, 

 or it may vary from point to point. In particular, it may depend 

 upon the magnitude of the field ; and the quality of one medium 

 may differ from that of another. Such variation of quality, 

 within one medium, or from one medium to another, is capable 

 of diagrammatic representation by a variation of the direction or 

 the strength of the field (other conditions being the same) from the 

 state manifested in some uniform medium taken as a standard. 

 The medium is said to be permeable to the force, in greater or less 

 degree than the standard medium, according as the variation of 

 the density of the lines of force from the standard case, under 

 otherwise identical conditions, is in excess or defect. A body 

 placed in the medium will tend to move toivards regions of greater or 

 less force according as its 'permeability is greater or less than that oj 

 the surrounding medium*. In the common experiment of placing 

 iron-fiHngs between the two poles of a magnetic field, the fihngs 

 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, were it not for friction or some other resistance, 

 they would soon gather together around the nearest pole. 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 surround- 

 ing, or adjacent, field. 



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



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

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

 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. (Cf. footnote, p. 187.) 



T. G. 12 



