404 THE FORMS OF CELLS [ch. 



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 cy Under 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 Aclilya or 

 in the young zygospore of Spirogyra, we 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 cyhndrical 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 aniso- 

 tropy of its envelope, which when more developed will convert the 

 ovoid into a cyhnder. We may also notice that a truly cyhndrical 

 cell is comparatively rare, for in many cases what we call a 

 cylindrical cell shews a distinct bulging of its sides; it is not truly 

 a cyhnder, but a portion of a spheroid or ellipsoid. 



Unicellular organisms in general — protozoa, unicellular crypto- 

 gams, various bacteria and the free isolated cells, spores, ova, etc. 

 of higher organisms — are referable for the most part to a small 

 number of typical forms ; but there are many others in which either 

 no symmetry is to be recognised, or in which the form is clearly 

 not one of equihbrium. Among these latter we have Amoeba itself 

 and all manner of amoeboid organisms, and also many curiously 

 shaped cells such as the Trypanosomes and various aberrant 

 Infusoria. We shall return to the consideration of these; but in 

 the meanwhile it will suffice to say (and to repeat) that, inasmuch 

 as their surfaces are not equihbrium-surfaces, so neither are the 

 Uving cells themselves in any stable equihbrium. On the contrary, 

 they are in continual flux and movement, each portion of the 



between chemical and histological structure, where micellae and long- chain molecules 

 enlarge and alter our conceptions not only of cellulose and keratin, but of pseudopodia 

 and cilia, of bone and muscle, and of the naked surface of the cell. See L. E. R. 

 Picken, The fine structure of biological systems, Biol. Reviews, xv, pp. 133-67, 1940. 



