446 



NATURE 



[October 6, 1910 



tures are also open to objection. The action of pepsin 

 and hydrochloric acid must depend very largely on the 

 accessibility of the material the character of which is to 

 be determined. If there are membranes protecting 

 cellular elements, pepsin, which is a colloid, if it diffuses 

 at all, must in some cases, at least, penetrate them with 

 difficulty. In Spirogyra, for example, the external mem- 

 brane, formed of a thick layer of cellulose, is impermeable 

 to pepsin, but not to the acid, and, in consequence, the 

 changes which occur in it during peptic digestion are due 

 to the acid alone. Even in the cell the periphery of 

 which is not protected by a membrane, the insoluble 

 colloid material at the surface serves as a barrier to the 

 free entrance of the pepsin. It is, however, more par- 

 ticularly in the action on the nucleus and its contents that 

 peptic digestion fails to give results which can be regarded 

 as free from objection. Here is a membrane which 

 during life serves to keep out of the nucleus, not only 

 all inorganic salts, but also all organic compounds, except 

 chiefly those of the class of nuclco-proteins. That such 

 a membrane may, when the organism is dead, be per- 

 meable to pepsin is at least open to question, and in 

 consequence what we see in the nucleus after the cell has 

 been acted on by pepsin and hydrochloric acid cannot be 

 adduced as evidence of its chemical or even of its morplio- 

 logical character. 



The results of digestive experiments on cells are, there- 

 fore, misleading. What may from them appear as nucleo- 

 protein may be anything but that, while, if the pepsin 

 penetrates as readily as the acid, there should be left, not 

 nucleo-protein, but pure nucleic acid, which should not 

 slain at all. 



The objections which I now urge against the con- 

 clusions drawn from the results of digestion experiments 

 have developed out of my own observations on veast cells, 

 diatoms, Spirogyra, and especially the blue-green algae. 

 The latter are, as is Spirogyra, encased in a membrane 

 which is an effective barrier to all colloids. When, there- 

 fore, threads of Oscillaria are subjected to the action of 

 artificial gastric juice, a certain diminution in volume is 

 observed owing to the dissolving power of .the hydro- 

 chloric acid, and an alteration of the staining power of 

 certain structures is found to obtain ; but the "pepsin has 

 nothing to do with these, as may be determined by 

 examination of control preparations treated with a solu- 

 tion of hydrochloric acid alone. 



It is thus seen how slender is our knowledge of the 

 chemistry of cells derived from staining methods and 

 from digestion experiments. That, however, has not been 

 the worst result of our confidence in our methods. It 

 has led cytologists to rely on these methods alone, to leave 

 undeveloped others which might have thrown great light 

 on the chemical constitution of the cell, and which might 

 have enabled us to understand a little more clearly the 

 causation of some of the vital phenomena. 



It was the futility of some of the old methods that led 

 me, twenty years ago, to attack the chemistry of the 

 cell from what appeared to me a correctly chemical point of 

 view. It seemed to me then, and it appears as true now, 

 that a diligent search for decisive chemical reactions would 

 yield results of the very greatest importance. In ' the 

 interval I have been able to accomplish only a small 

 fraction of what I hoped to do, but I think 'the results 

 have justified the view that, if there had been many 

 investigators in this line instead of only a very few, the 

 science of Cytochemistry would play a larger part in the 

 solution of the problems of cell physiology than it does 

 now. 



The methods and the results are, as I have said, meagre, 

 but they show distinctly indeed that the inorganic salts 

 are not diffused uniformly throughout the eel!, that in 

 vegetable cells they are rigidly localised, while in animal 

 cells, except those devoted to absorption and excretion, 

 they are confined to specified areas in the cell. Their 

 localisation, except in the case of inorganic salts of iron, 

 is not due to the formation of precipitates, but rather to 

 a condition which is the result of the action of surface 

 tension. This seems to me to be the only explanation 

 for the remarkable distribution, for example, of potash 

 salts in vegetable cells. We k-now that, except in the 

 chloroplatinate of potassium and in the hexanitrite of 



NO. 2136, VOL. S.ll 



potassium, sodium and cobalt, potassium salts form no 

 precipitates ; and yet, in the cytoplasm of vegetable cells, 

 the potassium is so localised at a few points as to appear 

 at first as if it were in the form of a precipitate. In 

 normal active cells of Spirogyra it is massed along the 

 edge of the chromotophor, while in the mesophylic cells 

 of leaves it is condensed in masses of the cytoplasm, which 

 are by no means conspicuous in ordinary preparations of 

 these cells. 



_ This effect of surface tension in localising the distribu- 

 tion of inorganic salts at points in the cytoplasm would 

 explain the distribution of potassium in motor structures. 

 In striated muscle the element is abundant in amount, and 

 is confined to the dim bands in the normal conditions. 

 In Vorticella, apart from a minute quantity present at a 

 point in the cytoplasm, it is found in very noticeable 

 amounts in the contractile stalk, while in the holotrichate 

 infusoria (ParamjEcium) it is in very intimate association 

 \vith the basal elements of the cilia in the ectosarc. 

 This, indeed, would seem to indicate that the distribution 

 of the potassium is closely associated with contraction, 

 and, therefore, with the production of energy in contrac- 

 tile tissues. The condensation of potassium at a point 

 may, of course, be a result of a combination with por- 

 tions of the cytoplasm, but we have no knowledge of 

 the occurrence of such compounds ; and, further, the 

 presence of such does not explain anything or account 

 for the liberation of energy in motor contraction. On 

 the other hand, the action of surface tension would ex- 

 plain, not only the localisation of the potassium, but also 

 the liberation of the energy. 



In vessels holding fluids, the latter, in relation to 

 surface tension, have two surfaces, one free, in contact 

 with the air, and known as the air-water surface, the 

 other that in contact with the wall of the containing 

 vessel (glass). In the latter the tension is lower than in 

 the former. When an inorganic compound — a salt, for 

 example — is dissolved in the fluid it increases the tension 

 at the air-water surface, but its dilution is much greater 

 here than in any other part of the fluid, while at the 

 other surface its concentration is greatest. In the latter 

 case the condition is of the nature of adsorption. The 

 condensation on that portion of the surface where the 

 tension is least is responsible for what we find when a 

 solution of a coloured salt, as, e.g., potassium perman- 

 ganate, is driven through a layer of dry sand. If the 

 latter is of some considerable thickness, the fluid as it 

 passes out is colourless. The air-solution surface tension 

 is higher than the tension of each of the solution-sand 

 surfaces, on which, therefore, the permanganate condenses 

 or is adsorbed. The same phenomenon is observed when 

 a long strip of filter paper is allowed to hang with its 

 lower end in contact with a moderately dilute solution 

 of a copper salt. The solution is imbibed by the filter 

 paper, and it ascends a certain distance in a couple of 

 minutes, when it may be found that the uppermost por- 

 tion of the moist area is free from even a trace of copper 

 salt. 



If, on the other hand, an organic compound — as, for 

 instance, one of the bile salts — instead of an inorganic 

 compound is dissolved in the fluid, the surface tension 

 of the air-water surface is reduced, and in consequence 

 the bile salt is concentrated at that surface, while in the 

 remainder of the fluid, and particularly in that portion 

 of it in contact with the wall of the vessel, the concentra- 

 tion is reduced. 



The distribution of a salt in such a fluid, whether it 

 lowers surface tension or increases it, is due to the action 

 of a law which may be expressed in words to the effect 

 that the concentration in a system is so adjusted as to 

 reduce the energy at any point to a minimum. 



Our knowledge of this action of inorganic and organic 

 substances on the surface tension in a fluid, and of the 

 differences in their concentrations throughout the latter, 

 was contained in the results of the observations on gas 

 mixtures by J. Willard Gibbs, published in 1878. The 

 principle as applied to solutions was independently dis- 

 covered by J. J. Thomson in 1887. It is known as the 

 Gibbs' principle, although the current enunciations of it 

 contain the more extended observations of Thomson. As 

 formulated usually it is more briefly given, and its 



