V] OF ASYMMETRY AND ANISOTROPY 243 



respect of the tension of the cell-wall. Now the former hypothesis 

 is not impossible ; the protoplasm is far from being a perfect fluid ; 

 it is the seat of various internal forces, sometimes manifestly 

 polar; and accordingly it is quite possible that the internal 

 forces, osmotic and other, which lead to an increase of the content 

 of the cell and are manifested in pressure outwardly directed 

 upon its wall may be unsymmetrical, and such as to lead to a 

 deformation of what would otherwise be a simple sphere. But 

 while this hypothesis is not impossible, it is not very easy of 

 acceptance. The protoplasm, though not a perfect fluid, has yet 

 on the whole the properties of a fluid; within the small compass 

 of the cell there is little room for the development of unsymmetrical 

 pressures ; and, in such a case as Spirogyra, where a large part of 

 the cavity is filled by a fluid and w^atery cell-sap, the conditions 

 are still more obviously those under which a uniform hydrostatic 

 pressure is to be expected. But in variations of T, that is to say 

 of the specific surface-tension per unit area, we have an ample 

 field for all the various deformations with which we shall have to 

 deal. Our condition now is, that [TjR + T'/R') = a constant ; but 

 it no longer follows, though it may still often be the case, that this 

 will represent a surface of absolute minimal area. As soon as T 

 and T' become unequal, it is obvious that we are no longer dealing 

 with a perfectly liquid surface film ; but its departure from a 

 perfect fluidity may be of all degrees, from that of a slight non- 

 isotropic viscosity to the state of a firm elastic membrane*. And 

 it matters little whether this viscosity or semi-rigidity be mani- 

 fested in the self-same layer which is still a part of the protoplasm 

 of the cell, or in a layer which is completely differentiated into a 

 distinct and separate membrane. As soon as, by secretion or 

 "adsorption," the molecular constitution of the surface layer is 

 altered, it is clearly conceivable that the alteration, or the secondary 

 chemical changes which follow it, may be such as to produce an 

 anisotropy, and to render the molecular forces less capable in 

 one direction than another of exerting that contractile force by 

 which they are striving to reduce to an absolute minimum the 



* Indeed any non-isotropic stiffness, even though T remained uniform, would 

 simulate, and be indistinguishable from, a condition of non-stiffness arid non- 

 isotropic T. 



16—2 



