406 



THE FORMS OF CELLS 



[CH. 



Many unicellular forms, and a few other simple organisms, are 

 spherical, and serve to illustrate in the simplest way the point at 

 issue. Unicellular algae, such as Protococcus or Halisphaera, the 

 innumerable floating eggs of fishes, the floating unilocular foraminifer 

 Orbulina, the lavely green multicellular Volvox of our ponds, all 

 these in their several grades of simplicity or complication are so 

 many round drops, spherical because no alien forces have deformed 

 or mis-shapen them. But observe that, with the exception of 

 Volvox, whose spherical body is covered wholly and uniformly with 

 minute ciha, all the above are passive or inactive forms; and in a 

 "resting" or encysted phase the spherical form is common and 

 general in a great range of unicellular organisms. 



Conversely, we see that those unicellular forms which depart 

 jUBrkedly from sphericity — excluding for the moment the amoeboid 



forms and those provided with skeletons 

 — are all cihate or flagellate. Ciha and 

 flagella are sui generis ; we know nothing 

 of them from the physical side, we cannot 

 reproduce or imitate them in any non- 

 hving drop or fluid surface. But we can 

 easily see that they have an influence on 

 form, besides serving for locomotion. 

 When our httle Monad or Euglena 

 develops a flagellum, that is in itself an 

 indication of asymmetry or "polarity," 

 of non-homogeneity of the little cell ; and 

 in the various flagellate types the flagellum or its analogues always 

 stand on prominent points, or ends, or edges of the cell — on parts, 

 that is to say, where curvature is high and surface-tension may be 

 expected to be low — for the product of surface-tension by mean 

 curvature tends to be constant. 



Fig. 123. A flagellate "monad," 

 Distigma proteus Ehr, 

 After Saville Kent. 



The minute dimensions of a cilium or a flagellum are such that the molecular 

 forces leading to surface-tension must here be under peculiar conditions and 

 restraints; we cannot hope to understand them by comparison with a whip- 

 lash, or through any other analogy drawn from a different order of magnitude. 

 I suspect that a ciliary surface is always electrically charged, and that 

 a point- charge is formed or induced in each cilium or flagellum. Just as we 

 learn the properties of a drop or a jet as phenomena proper to their scale of 

 magnitude, so some day we shall learn the very different physical, but 



