TROTOPLAS.MIC 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 CO, 

 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 cor])Uscle tends 

 to break down or undergo hcemolysis.^ This fact is of 



^ For data and references in this field, cf. Ilohcr's Physik. Chnnie 

 d. Zelle, pp. 247, 300, 4<S3, and 599. ]Morc recently the cata|)horcsis of 

 bacteria and their agglutination by electrolytes and oUicr substances has 

 been studied by Northrop and De Kruif (J. Gen. Physiol., IV (192:], 

 629, 639, and 655). • 



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



3 Michaelis and Takahashi, Biochcm. Zeitschrifl, XXIX (igio), 430- 

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

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



