PROTOPLASMIC STRUCTURE 99 



cell, is similarly bounded. Suspensions of living cells — 

 suspensions of blood corpuscles, eggs, spermatozoa, 

 bacteria, or yeast — in their normal aqueous media 

 may be regarded as similar in many respects to emulsions. 

 Each cell is a small discrete particle with a surface- 

 tension and an electrical potential-difference against its 

 medium. The existence of this potential is shown in 

 convection experiments; just as oil droplets migrate to 

 the anode in an electrical field, so do suspended living 

 cells; each cell, although living and highly differentiated 

 internally, behaves in this respect like a negatively charged 

 colloidal particle. As in the case of a suspended oil droplet, 

 the sign of the charge carried by the living particle may be 

 changed by acids or polyvalent ions; and like colloidal 

 particles in general the cells may be precipitated from 

 suspension (or agglutinated) under various conditions.^ 



Red blood corpuscles travel in neutral media like 

 isotonic sugar solution to the anode, thus showing the 

 presence of a negative surface charge; by passing CO3 

 through the media or adding weak acids, the sign of the 

 charge may be reversed; the corpuscles then become 

 positive or cathodic."^ At an intermediate concentration 

 the charge is abolished (isoelectric point); and it is 

 interesting to note that at this point the corpuscle tends 

 to break down or undergo •haemolysis.^ This fact is of 



^ For data and references in this field, cf. Hober's Physik. Chemie 

 d. Zelle, pp. 247, 300, 483, and 599. More recently the cataphoresis of 

 bacteria and their agglutination by electrolytes and other substances has 

 been studied by Northrop and De Kruif (/. Gen. Physiol., IV [1922], 

 629, 639, and 655). 



^ Hober, Arch. ges. Physiol., CI (1904), 627; CII, 196. 



3 Michaelis and Takahashi, Biochem. Zeitschrift, XXIX (1910), 439. 

 The isoelectric point is also the optimum for agglutination; cf. C. R. 

 Coulter, Jour. Gen. Physiol., Ill (1920), 309. 



