24 



THE CELL AND PROTOPLASM 



with a motor, and the degree of the pres- 

 sure is determined by a manometer. 



The cell and the oil drop behave in a 

 manner similar to two liquid drops in con- 

 tact. Such a system may or may not be 

 stable, depending, among other things, on 

 the magnitude of the tension and the di- 

 ameter of the oil drop. If the system is 

 unstable, the two drops will spontaneously 

 coalesce and produce a stable equilibrium. 

 In the case of the Arlacia egg, if coales- 

 cency occurs at all, it has been found that 

 the cell always incorporates the oil. This 

 reaction is rapid, taking less time than the 

 interval between two successive frames of 

 a motion picture film at sixteen frames per 

 second. 



The tendencies toward coalescency are 

 proportional to the tension at the interface 

 of the oil-aqueous phase and to the diameter 

 of the freshly applied oil drop. The im- 

 portance of tension and the size of oil drop 

 (i.e., surface area) suggest reactions in- 

 volving changes in surface energies. Coa- 

 lescence is therefore thermodynamically 

 possible since the potential energy of the 

 cell with the oil drop inside is lower than 

 with the oil drop outside. 



The level of potential energy of the cell 

 and the oil drop in contact with it may be 

 regulated either by selecting an oil having 

 a high i?aterfacial tension against the aque- 

 ous phase or by increasing the size of the 

 applied drop. 



Frequently the potential energy of the 

 oil in contact with the cell has to be very 

 high before the oil will be engulfed by the 

 cell. This would not be so if the cell were 

 an ideal liquid drop, since liquid drops 

 coalesce spontaneously with only infinitesi- 

 mal differences in the potential energies of 

 the two. The relative pronounced differ- 

 ences indicate the presence of a potential 

 hill at the cell surface. The magnitude of 

 this potential hill may be calculated by 

 determining the critical diameter of the oil 

 drop at the moment of coalescence. A de- 

 termination of the relative differences of 

 coalescency is of considerable value for 

 ascertaining the nature of the surfaces of 

 different cells and of cells under experi- 

 mental treatment. Kopac has made a care- 



ful quantitative study of this problem and 

 has compared the potential hill of the ex- 

 perimental cells with that of normal cells, 

 selected as control cells. 



These data may be represented as useful 

 ratios as follows: 



Potential energy (control) _ 



Potential energy (experimental) 



relative coalescency. 



Thus, ratios greater than 1 indicate that 

 potential hills of the experimentally treated 

 cells are lower than those of the control 

 cells. 



A high value of the potential hill at the 

 cell surface may be due to a tangential 

 rigidity at the cell surface, or to the exis- 

 tence of a third phase between the cell 

 surface and the oil drop, which is either 

 tangentially rigid or possesses surface 

 activity. 



The coalescency method has been very 

 useful in studying the problem of extrane- 

 ous coats in cells. Kopac and Chambers 

 (1938) have made a series of determina- 

 tions on the effect of removing the vitelline 

 membrane of the Arhacia egg by various 

 means. 



The control egg was the unfertilized egg, 

 washed and centrifuged several times in 

 isosmotic sodium chloride to remove the 

 jelly but not its vitelline membrane. The 

 relative coalescency value for these control 

 eggs was taken as 1. A relatively high coa- 

 lescency value of 7 to 15 was obtained on 

 eggs within three minutes after fertiliza- 

 tion, the fertilization membranes having 

 been removed by shaking immediately after 

 insemination. This indicates that the proc- 

 ess of fertilization has definitely lowered 

 the potential hill, presumably by removing 

 the vitelline membrane during the forma- 

 tion of the fertilization membrane. The 

 coalescency value of fertilized eggs remain- 

 ing in sea water progressively drops, be- 

 cause, as will be shown later, a hyaline 

 layer begins to be excreted on the surface 

 of the egg which attains pronounced pro- 

 portions six to eight minutes after fertili- 

 zation. However, if the fertilized eggs are 

 placed in a solution of potassium chloride 

 isosmotic with sea water, the presence of 



