VI] OF MACALLUM'S EXPERIMENTS 463 



And now to return, for a moment, to the question of cell-form. 

 When we assert that the form of a cell (in the absence of mechanical 

 pressure) is essentiall> ..^pendent on surface-tension, and even when 

 we make the preliminary assumption that protoplasm is essentially 

 a fluid, we are resting our belief on a general consensus of evidence, 

 rather than on comphance with any one crucial definition. The 

 simple fact is that the agreement of cell-forms with the forms which 

 physical experiment and mathematical theory assign to liquid 

 surfaces under the influence of surface-tension is so frequently and 

 often so typically manifested that we are led, or driven, to accept 

 the surface-tension hypothesis as generally apphcable and as equi- 

 valent to a universal law. The occasional difficulties or apparent 

 exceptions are such as to call for further enquiry, but fall short of 

 throwing doubt on the hypothesis. Macallum's researches introduce 

 a new element of certainty, a "nail in a sure place," when they 

 demonstrate that in certain movements or changes of form which 

 we should naturally attribute to weakened surface-tension, a 

 chemical concentration which would naturally accompany such 

 weakening actually takes place. They further teach us that in the 

 cell a chemical heterogeneity may exist of a very marked kind, 

 certain substances being accumulated here and absent there, within 

 the narrow bounds of the system. 



" Such localised accumulations can as yet only be demonstrated in 

 the case of a very few substances, and of a single one in particular ; 

 and these few are substances whose presence does not produce, but 

 whose concentration tends to follow^ a weakening of surface-tension. 

 The physical cause of the localised inequalities of surface-tension 

 remains unknown. We may assume, if we please, that they are 

 due to the prior accumulation, or local production, of bodies which 

 have this direct effect; though we are by no means limited to this 

 hypothesis. But in spite of some remaining difficulties and un- 

 certainties, we have arrived at the conclusion, as regards unicellular 

 organisms, that not only their general configuration but also their 



is part of the general subject of ionic regulation, which has since become a matter 

 of great physiological importance; cf. [int. al.) D. A. Webb, Ionic regulation in 

 Carcinus mnenns, Pror. R,S. (B), cxxix, pp. 107-130, 1940, and many works 

 quoted therein. It is curious and interesting that Macallum's first work on unequal 

 ionic distribution in the tissues and Donnan's fundamental conception of the Donnan 

 equilibrium {Journ. Chem,. Sdv. xcix, p. 1554, 1911) came just at the same time. 



