IV] AND STRUCTURE OF THE CELL 297 



The morphologist is accustomed to speak of a "polarity" of the 

 cell, meaning thereby a symmetry of visible structure about a 

 particular axis. For instance, whenever we can recognise in a cell 

 both a nucleus and a centrosome, we may consider a hne drawn 

 through the two as the morphological axis of polarity ; an epitheUum 

 cell is morphologically symmetrical about a median axis passing 

 from its free surface to its attached base. Again, by an extension 

 of the term polarity, as is customary in dynamics, we may have 

 a "radial" polarity, between centre and periphery; and lastly, we 

 may have several apparently independent centres of polarity within 

 the single cell. Only in cells of quite irregular or amoeboid form 

 do we fail to recognise a definite and symmetrical polarity. The 

 morphological polarity is accompanied by, and is but the outward 

 expression (or part of it) of a true dynamical polarity, or distribution 

 of forces; and the hues of force are, or may be, rendered visible 

 by concatenation of particles of matter, such as come under the 

 influence of the forces in action. 



When hnes of force stream inwards from the periphery towards 

 a point in the interior of the cell, particles susceptible of attraction 

 either crowd towards the surface of the cell or, when retarded by 

 friction, are seen forming lines or "fibrillae" which radiate outwards 

 from the centre. In the cells of columnar or cihated epithehum, 

 where the sides of the cell are symmetrically disposed to their 

 neighbours but the free and attached surfaces are very diverse from 

 one another in their external relations, it is these latter surfaces 

 which constitute the opposite poles; and in accordance with the 

 parallel lines of force so set up, we very frequently see parallel lines 

 of granules which have ranged themselves perpendicularly to the 

 free surface of the cell (cf. Fig. 149). 



A simple manifestation of polarity may be well illustrated by 

 the phenomenon of diffusion, where we may conceive, and may 

 automatically reproduce, a field of force, with its poles and its 

 visible lines of equipotential, very much as in Faraday's conception 

 of the field of force of a magnetic system. Thus, in one of Leduc's 

 experiments*, if we spread a layer of salt solution over a level 

 plate of glass, and let fall into the middle of it a drop of indian 

 ink, or of blood, we shall find the coloured particles travelling 

 * Thtorie physico-chimique de la Vie, 1910, p. 73. 



