V] OF ASYMMETRY AND ANISOTROPY 241 



This subject we shall discuss later, in connection with cell- 

 aggregates or tissues, and we shall find that further theoretical 

 considerations are needed as a preliminary to any such enquiry. 

 Meanwhile let us consider a few cases of the forms of cells, either 

 solitary, or in such simple aggregates that their individual form is 

 little disturbed thereby. 



Let us clearly understand that the cases we are about to 

 consider are those cases where the perfect symmetry of the sphere 

 is replaced by another symmetry, less complete, such as that of 

 an elhpsoidal or cyUndrical cell. The cases of asymmetrical 

 deformation or displacement, such as is illustrated in the production 

 of a bud or the development of a lateral branch, are much simpler. 

 For here we need only assume a sUght and locaHsed variation of 

 surface-tension, such as may be brought about in various ways 

 through the heterogeneous chemistry of the cell; to this point 

 we shall return in our chapter on Adsorption. But the diffused 

 and graded asymmetry of the system, which brings about for 

 instance the elhpsoidal shape of a yeast-cell, is another matter. 



If the sphere be the one surface of complete symmetry and 



therefore of independent equilibrium, it follows that in every cell 



which is otherwise conformed there must be some definite force 



to cause its departure from sphericity ; and if this cause be the 



very simple and obvious one of the resistance offered by a solidified 



envelope, such as an egg-shell or firm cell-wall, we must still seek 



for the deforming force which was in action to bring about the 



given shape, prior to the assumption of rigidity. Such a cause 



may be either external to, or may lie within, the cell itself. On 



the one hand it may be due to external pressure or to some form 



of mechanical restraint: as it is in all our experiments in which 



we submit our bubble to the partial restraint of discs or rings or 



more complicated cages of wire ; and on the other hand it may be 



due to intrinsic causes, which must come under the head either of 



differences of internal pressure, or of lack of homogeneity or 



isotropy in the surface itself*. 



* A case which we have not specially considered, but which may be found to 

 deserve consideration in biology, is that of a cell or drop suspended in a liquid of 

 varying density, for instance in the upper layers of a fluid (e.g. sea-water) at whose 

 surface condensation is going on, so as to produce a steady density-gradient. In 

 this case the normally spherical drop will be flattened into an oval form, with its 



T. G, 16 



