400 THE FORMS OF CELLS [ch. 



needed as a preliminary to any such enquiry. Meanwhile let us 

 consider a few cases of the forms of cells, either sohtary, or in such 

 simple aggregates that their individual form is httle disturbed thereby. 

 Let us clearly understand that the cases we are about to consider 

 are those where the perfect sjnnmetry of the sphere is replaced by 

 another symmetry, less complete, such as that of an ellipsoidal or 

 cylindrical cell. The cases of asymmetrical deformation or dis- 

 placement, such as are illustrated in the production of a bud or 

 the development of a lateral branch, are much simpler; for here 

 we need only assume a slight and locahsed variation of surface- 

 tension, such as may be brought about in various ways through 

 the heterogeneous chemistry of the cell. But such diffused and 

 graded asymmetry as brings about for instance the ellipsoidal 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 cause of 

 its departure from sphericity; and if this cause be the obvious one 

 of 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 some form of mechanical restraint, as when we submit our 

 bubble to the partial restraint of discs or rings or more compHcated 

 cages of wire ; on the other hand it maybe 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 or its envelope*. 



* 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 

 maximum surface-curvature lying at the level where the densities of the drop 

 and the surrounding liquid are just equal. The sectional outline of the drop has 

 been shewn to be not a true oval or ellipse, but a somewhat complicated quartic 

 curve. (Rice, Phil. Mag. Jan. 1915.) A more general case, which also may well 

 deserve consideration by the biologist, is that of a charged bubble in (for instance) 

 a uniform field of force : which will expand or elongate in the direction of the lines 

 of force, and become a spheroidal surface in continuous transformation with the 

 original sphere. 



