288 PROTOPLASM 



calcium, in the proportion of 100 molecules of the former to 

 1 or 2 of the latter, which is approximately the proportion in 

 which these elements occur in sea water and in blood, balance 

 one another. The mixture has no effect on emulsions. Clowes 

 viewed the plasma membrane as a fine emulsion; when oil is 

 dispersed in water, the membrane is more permeable (to water- 

 soluble substances, i.e., salts); when water is dispersed in oil, 

 the membrane is less permeable. The former condition is pre- 

 sumably produced by sodium, and the latter by calcium. Nor- 

 mally, the protoplasmic emulsion is in a state of equilibrium 

 near the reversal point, for it is bathed in a balanced solution. 

 It is, therefore, readily thrown one way or the other by a change 

 in concentration of the salts in the surrounding medium (Fig. 83). 



The best support of Clowe's hypothesis is the fact that most 

 oil emulsions are at the reversal point when sodium and calcium 

 are present in the same proportion as they occur in physiologically 

 balanced solutions. The hypothesis met with adverse criticism 

 on the grounds that there is no evidence that the protoplasmic 

 membrane is a fine emulsion, and it is the hydroxyl (0H~) anion 

 rather than the sodium cation which reverses and holds emulsions 

 in the oil-in-water state. In reply to these criticisms, the English 

 botanists Dixon and Clark said that an electrical stimulus 

 affects emulsions in the same way as it does protoplasm. An 

 electric current will cause an emulsion, originally almost imper- 

 meable to ions and water-soluble substances, to become fairly 

 permeable, which is the same effect that electric stimuli have on 

 living tissues; viz., they increase permeability. Dixon and Clark, 

 therefore, conclude that a hypothesis of the structure of the 

 plasmatic membrane that explains two such apparently uncon- 

 nected and remarkable phenomena as the antagonistic action 

 of certain ions on permeability (the hypothesis of Clowes) and 

 the permeability changes produced by electric stimuli (the work 

 of Dixon and Clark) deserves serious consideration. This is 

 true, yet it may simply mean that two rather diverse types of 

 systems (an emulsion and a living jelly) show similar responses 

 to the same environmental changes. 



We are forced to discard the emulsion hypothesis of membrane 

 control in spite of two substantial facts in its support (that of 

 Clowes and that of Dixon just cited) because of the following 

 reasons: There is no direct evidence whatever of phase reversal 



