FIELDS OF FORCE 381 



Its surface-volume ratio is 3/1. Doubling the radius increases 

 the surface four times while increasing the volume eight times, 

 i.e. SjV = |, i.e. the effective surface has been halved. The 

 immediate result of decreasing the specific surface to a value below the 

 minimal effective value is to decrease the supply of oxygen to the centre 

 of the cell and to cause a heaping up of carbon-dioxide and other 

 products of metabolic activity. This has, at least, two effects 

 () The process of development is retarded (Law of balanced 

 reactions), (b) The protoplasm becomes acid. The effect of acid 

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

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

 inquire into the reason for the symmetrical 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 

 separation 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 division is accompanied by a complex 

 but regular series of changes in the distribution of the nuclear 

 chromatin. These mitotic or karyokinetic changes are dependent 

 on the bipolarity 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 bipolar cell there are two " 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. XVI.) 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 

 themselves. Friction prevents the filings from collecting in mass round 

 the poles while the specific inductive capacity (p. 52) 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 



