240 COLLOIDS IN BIOLOGY AND MEDICINE 



quently surrounded by a visible and solid membrane which is usually 

 regarded as semipermeable. 



The basis of this view is: if such a cell is placed in hypertonic 

 salt solution, the protoplasm retracts from the cell wall and water is 

 lost. This phenomena is called plasmolysis. If the cell is im- 

 mersed in pure water, the protoplasm swells up again. The phe- 

 nomenon was formerly explained by saying that the membrane was 

 impermeable for salts. 1 In a hypertonic salt solution, water may in- 

 deed leave the cell but salt cannot enter; in pure water the process 

 is reversed. 



Nowadays discussion is focussed on the nature of the plasma 

 pellicle and two main tendencies may be recognized. 



Among the adherents to the lipoid theory in addition to E. OVERTON, 

 is VERNON, who considers it probable that the lipoid membrane 

 penetrates the interior of the cell. J. LOEB and R. BEUTNER in view 

 of their investigations of bio-electric phenomena may be regarded as 

 adherents of the lipoid theory. Those investigators (J. TRAUBE and 

 F. CZAPEK) who regard changes in surface tension as the means of 

 penetrating surfaces may be regarded as adherents of a modified 

 lipoid membrane theory. They arrive at this conclusion because 

 their experiments have been chiefly concerned with the action of 

 lipoid soluble substances on the cell. 



According to F. CZAPEK all substances whose surface tension is 

 less than 0.68 (water /air = 1) are toxic for the higher plant cells and 

 CZAPEK'S pupil KISCH determined 0.5 to be the limit of toxic surface 

 tension for yeast cells and fungi. Since lecithin and cholesterin, that 

 is, the lipoids and their emulsions, have a surface tension of 0.5, 

 F. CZAPEK agrees with NATHANSON and regards the cell membrane 

 as a concentrated fat-emulsion which is permeable for either fat or for 

 water soluble substance depending on the conditions of surface ten- 

 sion. Similar views (loose union of albumin and lipoid) are enter- 

 tained by W. W. LEPESCHKIN with the difference that he regards the 

 entire protoplasm as such an emulsion possessing properties hi the 

 center similar to those on the surfaces. 



The " emulsion theory" obtained very definite support from the fol- 

 lowing observation of CLOWES (see p. 38) . He prepared an oil-water 

 emulsion by shaking equal quantities of water and olive oil and suffi- 

 cient n/10 NaOH that the outer phase (the water) was just alkaline 

 to phenolphthalein. If he now added a small excess of CaC^ solution 

 the emulsion changed into a water-oil emulsion; in other words, 

 water became the dispersed phase in a continuous layer of oil. We 



1 The visible cell membrane is quite permeable for most crystalloids, serving 

 only to a certain extent as a support for the protoplasm. 



