244 THE FORMS OF CELLS [ch. 



surface area of the cell. A slight inequality in two opposite 

 directions will produce the ellipsoid cell, and a very great in- 

 equality will give rise to the cyHndrical cell*. 



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

 any other cylindrical 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 

 surface-membrane; and that this molecular constitution was 

 anisotropous, in such a way as to render extension easier in one 

 direction than another. 



Such a lack of homogeneity or of isotropy, in the cell-wall is 

 often rendered visible, especially in plant-cells, in various ways, 

 in the form of concentric lamellae, annular and spiral striations, 

 and the like. 



But this phenomenon, while it brings about a certain departure 

 from complete symmetry, is still compatible with, and coexistent 

 with, many of the phenomena which we have seen to be associated 

 with surface-tension. The symmetry of tensions still leaves the 

 cell a solid of revolution, and its surface is still a surface of equi- 

 librium. The fluid pressure within the cylinder still causes the 

 film or membrane which caps its ends to be of a spherical form. 

 And in the young cell, where the surface pellicle is absent or but 

 little differentiated, as for instance in the oogonium of Achlya, 

 or in the young zygospore of Spirogyra, we always see the tendency 

 of the entire structure towards a spherical form reasserting itself : 

 unless, as in the latter case, it be overcome by direct compression 

 within the cylindrical mother-cell. Moreover, in those cases 

 where the adult filament consists of cylindrical cells, we see that 

 the young, germinating spore, at first spherical, very soon assumes 

 with growth an elliptical or ovoid form : the direct result of an 

 incipient anisotropy of its envelope, which when more developed 

 will convert the ovoid into a cyHnder. We may also notice that 

 a truly cylindrical cell is comparatively rare; for in most cases, 

 what we call a cylindrical cell shews a distinct bulging of its sides ; 

 it is not truly a cylinder, but a portion of a spheroid or ellipsoid. 



* A non-symmetry of T and T' might also be capable of explanation as a result 

 of "liquid crystallisation." This hypothesis is referred to, in connection with the 

 blood-corpuscles, on p. 272. 



