398 THE FORMS OF CELLS [ch. 



their part, under balanced conditions of temperature, density and 

 chemical composition. 



A little green infusorian from the Baltic Sea is, as near as may 

 be, a medusa in miniature*. It is curious indeed to find the same 

 medusoid, or as we may now call it vorticoid, configuration occurring 

 in a form so much lower in the scale, and so much less in order of 

 magnitude, than the ordinary medusae. 



According to Plateau, the viscidity of the liquid, while it 

 retards the breaking up of the cylinder and increases the length 

 of the segments beyond that which theory demands, has never- 

 theless less influence in this direction than we might have expected. 

 On the other hand any external support or adhesion, or mere 

 contact with a sohd body, will be equivalent to a reduction of 

 surface-tension and so will very greatly increase the stability of 

 our cylinder. It is for this reason that the mercury in our thermo- 

 meters seldom separates into drops: though it sometimes does so, 

 much to our inconvenience. And again it is for this reason that 

 the protoplasm in a long tubular or cyhndrical cell need not divide 

 into separate cells and internodes until the length of these far 

 exceeds the theoretical limits. 



An interesting case is that of a viscous drop immersed in another 

 viscous fluid, and drawn out into a thread by a shearing motion of 

 the latter. The thread seems stable at first, but when left to rest 

 it breaks up into drops of a very definite and uniform, size, the size 

 of the drops, or wave-length of the unduloid of which they are made, 

 depending on the relative viscosities of the two threads f- 



Plateau's results, though discovered by way of experiment and 

 though (as we have said) they illustrate the " materiahsation " of 

 mathematical law, are nevertheless essentially theoretical results 

 approached rather than realised in material systems. That a hquid 

 cylinder begins to be unstable when its length exceeds ^-nr is all 

 but mathematically true of an all but immaterial soap-bubble ; but 

 very far from true, as Plateau himself was well aware, in a flowing 

 jet, retarded by viscosity and by inertia. The principle is true and 

 universal; but our living cylinders do not follow the abstract laws 



* Medusachloris phiale, of A. Pascher, Biol. Centralbl. xxxvii, pp. 421-429, 1917. 



t See especially Rayleigh, Phil. Mag. xxxiv, p. 145, 1892, by whom the subject 

 is carried much further than where Plateau left it. See also {int. al.) G. I. Taylor, 

 Proc. R.S. (A), cxLvi, p. 501, 1934; S. Tomotika, ibid, cl, p. 322, 1935; etc. 



