CELL DIVLSION 481 



on an alkaline gel-emulsion has already been eonsidered and may 

 possibly be the cause of cell dixision. We have, however, to 

 inquire into the reason for the sijiiiiiu'trical division of the cell. 



Some cells divide directly without showing mitotic figures. 

 After a change in the distribution of free energy manifested by a 

 division of the nucleolus into two separate nucleoli, the whole 

 nucleus is divided equally into two daughter nuclei. This separa- 

 tion is followed by the formation of a cell membrane between the 

 two nuclei dividing the entire cell into two equal and similar 

 portions. 



Usually, however, cell di\ision is accompanied by a complex 

 but regular series of changes in the distribution of the nuclear 

 chromatin. These mitotic or karijokinetic changes are dependent 

 on the bipolaritij of the cell. Morphologically the polarity of 

 a cell refers to a symmetry of visible structure about a particular 

 axis. For instance, a line, drawn through the centres of nucleus 

 and centrosome, symmetrically divides a typical resting cell and 

 may be considered as its axis of polarity. This symmetry of form, 

 is an indication or expression of a symmetry of free energy. 



In a l)ipolar cell there are tw^o " poles " or centres of force, 

 and the axis of symmetry must divide the field of force equally 

 about these poles. 



Typical fields of force may be plotted by scattering iron filings on a sheet 

 of glass resting on the pole (monopolar field) or poles (bipolar field) of a 

 magnet. The filings set themselves along the lines of force, each little 

 filing becoming polarised and exerting an influence on adjacent filings. 

 (This ■' carding out " under the influence of stress was dealt with in Chap. 

 XVII.) In addition to the strength of the field and the direction of the 

 force, the movement of particles under its influence depends on the friction 

 of the contiguous matter and on the chemical nature of the particles them- 

 selves. Friction prevents the filings from collecting in mass round the poles 

 while the specific inductive capacity (q.v.) or "' permeability " of the particles 

 is a measure of the influence exerted by the " force " on the particle. In the 

 case of magnetic force, the specific inductive capacity of iron is high while 

 that of bismuth is low. Iron filings will, therefore, be attracted towards 

 the poles and will tend to lie on the lines of stress. On the other hand, 

 bismuth filings are polarised in a sense opposite to that of the adjacent 

 field. They are forced by the incidence of stress to move (or because of 

 friction, to tend to move) from the stronger to the weaker parts of the field 

 and thus take up positions as far from the poles as possible. In general, a 

 body placed in a field of force will tend to move toivards regions of greater or less 

 ifitensity of stress according as its " pernteability " to the particular form of 

 energy in question is greater or less than that of the surrounding medium. 



The introduction of an aggregate of high permeability into the field of 

 force makes considerable changes in its configuration. Suppose a small 

 heap of filings were placed in the magnetic field already referred to, so that 

 it lay somewhat out of the interpolar axis but on the equatorial axis, the 

 result would be to provide an " easier path "' for the lines of force. It is 



Bi 31 



