IV] STRUCTURE OF THE CELL 167 



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 

 line drawn through the two as the morphological axis of polarity : 

 in an epithehum cell, it is obvious that the 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 morpho- 

 logical "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 rendered visible by con- 

 catenation of particles of matter, such as come under the influence 

 of the forces in action. 



When the lines of force stream inwards from the periphery 

 towards a point in the interior of the cell, the 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 and constitute a so-called 

 "aster." In the cells of columnar or ciliated epithelium, 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. 97). 



A simple manifestation of "polarity" may be well illustrated 

 by the phenomenon of diffusion, where w^e may conceive, and may 

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

 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 



* Theorie physico-chimique de la Vie, p. 73, 1910; Mechanism of Life, p. 56, 

 1911. 



