402 THE FORMS OF CELLS [ch. 



cell and are manifested in pressure outwardly directed upon its wall 

 niay be unsymmetrical, and such as to deform 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 httle room for the development 

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

 most part of the cavity is filled by watery sap, the conditions are 

 still more obviously, or more nearly, those under which a uniform 

 hydrostatic pressure should be displayed. 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 (TIR + T' jR) = a con- 

 stant; 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, we are no longer dealing 

 with a perfectly hquid surface film; but its departure from 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 manifested 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 a minimum the surface area of the cell. A slight inequality in 

 two opposite directions will produce the eUipsoid cell, and a great 

 inequahty will give rise to the cylindrical cell. 



I take it therefore, that the cylindrical cell of Spirogyra, or any 

 other cyhndrical cell which grows in freedom from any manifest 

 external restraint, has assumed that particular form simply by 

 reason of the molecular constitution of its developing wall or 



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

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

 isotropic T. 



