PROTOPLASMIC PERMEABILITY 145 



difficulty. Manifestly, shrinkage or swelling of the globules 

 will affect greatly the ease of entry of substances soluble in the 

 medium. Such shrinkage or swelling will follow changes in the 

 distribution of water between the phases, and, if protoplasm is 

 analogous to simpler colloids, such alterations of water distribu- 

 tion can be caused by changes in electrolyte concentration or by 

 other quite possible causes. No further detail is needed to indi- 

 cate the possibility of causing by such a mechanism the changes 

 of permeability which are actually observed. 



It should be noted that solubility in the globule phase alone 

 will not enable entry into the cell. The globules are isolated. 

 In order to pass from one globule to another a dissolved substance 

 must pass through a layer of medium. No substance can do this 

 unless it is soluble in the medium phase as well as in the glob- 

 ule phase. It follows that a substance which always penetrates 

 readily must do so either because it is soluble in both phases or 

 because, being soluble in the medium only, it causes the globules 

 to shrink, thus widening the passages for its entry. Doubtless 

 water enters in the first way and this is probably true also of sub- 

 stances like urea, glycerine and the monatomic alcohols which 

 appear to enter most cells very readily and regardless of the con- 

 dition of the cell or the presence of other substances. Sodium 

 chloride also enters readily when antagonistic salts are absent 

 but it probably enters in the second way, that is by causing shrink- 

 age of the globules. This is indicated by the fact that it also 

 increases permeability for other substances as was shown, for 

 instance, by Lillie for the pigment of Arenicola larvae.^" 



If a substance enters the cell only at times, as the amino acids 

 and most salts appear to do, that is because it does not cause the 

 globules to shrink and is therefore able to enter only when the 

 condition of the cell or the effect of some other agent has caused 

 sufficient shrinkage to provide a path adequate for entry. If a 

 substance impedes the entry of another, as calcium chloride does 

 for sodium chloride or for most dyes, that is because it has caused 

 the globules to swell and thus to close more completely the paths 

 between them. The explanation of antagonism is obvious. 



10 Amer. Jour. Physiol. 24: 23 (1909); 28: 211 (1911). 



