IV] STRUCTURE OF THE CELL 165 



history." On simple dynamical gromids, the contrast is easily 

 explained. So long as the fluid substance of the nucleus is quah- 

 tatively different from, and incapable of mixing with, the fluid 

 or semi-fluid protoplasm which surrounds it, we shall expect it 

 to be, as it almost always is, of spherical form. For, on the one 

 hand, it is bounded by a liquid film, whose surface-tension is 

 uniform ; and on the other, it is immersed in a medium which 

 transmits on all sides a uniform fluid pressure *. For a similar 

 reason the contractile vacuole of a Protozoon is spherical in form : 

 it is just a "drop" of fluid, bounded by a uniform surface- 

 tension and through whose boundary-film diffusion is taking place. 

 But here, owing to the small difference between the fluid constitut- 

 ing, and that surrounding, the drop, the surface-tension equi- 

 librium is unstable ; it is apt to vanish, and the rounded outline 

 of the drop, like a burst bubble, disappears in a moment t- 

 The case of the spherical nucleus is closely akin to the spherical 

 form of the yolk within the bird's egg J. But if the substance of 

 the cell acquire a greater solidity, as for instance in a muscle 



* "Souvent il n'y a qu'une separation physique entre le cytoplasme et le sue 

 nucleaire, comme entre deux liquides immiscibles, etc.;" Alexeieff, Sur la mitosc 

 dite ''primitive," Arch. f. Protistenk. xxix, p. 357, 1913. 



f The aj^pearance of " vacuolation " is a result of endosmosis or the diffusion 

 of a less dense fluid into the denser plasma of the cell. Caeferis paribus, it is less 

 apparent in marine organisms than in those of freshwater, and in many or most 

 marine Ciliates and even Rhizopods a contractile vacuole has not been observed 

 (Biitschli, in Bronn's Protozoa, p. 1414) ; it is also absent, and probably for the same 

 reason, in parasitic Protozoa, such as the Gregarines and the Entamoebae. Rossbach 

 shewed that the contractile vacuole of ordinary freshwater Ciliates was very greatly 

 diminished in a 5 per cent, solution of NaCl, and all but disappeared in a 1 per cent, 

 solution of sugar {Arb. z. z. Inst. Wurzburg, 1872, of. Massart, Arch, de Biol, lx, 

 p. 515, 1889). Actmophrys sol, when gradually acclimatised to sea-water, loses its 

 vacuoles, and vice versa (Gruber, Biol. Centralbl. ix, p. 22, 1889) ; and the same is 

 true of Amoeba (Zuelzer, Arch. f. Entw. Mech. 1910, p. 632). The gradual enlarge- 

 ment of the contractile vacuole is precisely analogous to the change of size of a 

 bubble until the gases on either side of the film are equally diffused, as described 

 long ago by Draper {Phil. Mag. (n. s.), xi, p. 559, 1837). Rhumbler has shewn 

 that contractile or pulsating vacuoles may be well imitated in chloroform-drops, 

 suspended in water in which various substances are dissolved {Arch. f. Entw. 

 Mech. VII, 1898, p. 103). The pressure within the contractile vacuole, always 

 greater than without, diminishes with its size, being inversely proportional to 

 its radius; and when it lies near the surface of the cell, as in a Heliozoon, it 

 bursts as soon as it reaches a thinness which its viscosity or molecular cohesion no 

 longer permits it to maintain. 



t Cf, p. 660. 



