W. O. FENN 867 



that the cell was so fluid at this temperature that it could pull away, leaving a thin 

 film behind. For the purposes of this comparison, quartz and glass surfaces are con- 

 sidered identical, which may not be strictly true. Some of these discrepancies between 

 observation and the surface tension theory may also be explained by the fact that 

 the changes of shape demanded of the cell for phagocytosis and spreading are very 

 different. Moreover, in the methods which have been used for measuring stickiness, 

 consisting in inverting the surface or in exposing it to a constant flow of water, the 

 force acting to disengage the particle from the surface is greater (at least different) 

 from the forces tending to disengage a particle about to be ingested. 



I have attempted to determine experimentally whether the rate of phagocytosis 

 was dependent in any way upon the size of the particles. Only particles up to 5 micra 

 in diameter were used, and no differences could be detected except such as were due 

 to the greater availability of the larger particles (greater chance of collision). Certain 

 experiments did lead to the belief that very small particles (i micron in diameter) 

 were not readily ingested, but the difficulty of counting these very small particles 

 makes this result seem now rather doubtful. In this respect, therefore, the theory is 

 in agreement with such inadequate experiments as have been performed. Particle 

 size would, of course, become important if the capacity of the cells became a limiting 

 factor, 



STICKINESS OF CELLS 



Whether or not the theory outlined above is adequate to cover the subject of 

 phagocytosis, it would seem to be adequate to provide a scientific definition of the 

 word "stickiness" which is often confused with viscidity. For a particle to stick to a 

 cell the contact angle must be less than 180°. Water makes such an angle (0°) with 

 glass, and would be difficult to remove from glass if it did not tear away, leaving a 

 thin film behind. Most substances commonly called "sticky," such as glue, owe this 

 property to the fact that they contain water which forms a small angle of contact with 

 solid substances. To take advantage of this property of water, something, such as 

 gelatin, must be added to it to increase its consistency so that it will not tear away. 

 Similar changes in water can be produced by freezing, in which case the ice adheres 

 firmly. The viscidity of glue is independent of the solid phase with which it is in con- 

 tact, but the stickiness of water depends upon interfacial surface tensions and is, there- 

 fore, dependent upon the nature of the solid phase. In defining a cell as "sticky," 

 therefore, it is proper to ask, "Sticky to what?" 



Some persons persist in attributing the behavior of cells in solution to their 

 "stickiness" because this seems "simpler" than to attribute it to surface tension. The 

 supposed difficulties of surface tension cannot, however, be avoided by this con- 

 venient subterfuge, for whether the force which causes the cells to adhere be described 

 as "adhesion," "cohesion," "electrical," "chemical," or simply as "stickiness," the 

 fact that they do adhere is nevertheless accurately described thermodynamically by 

 the further fact that their separation would involve an increase in the free energy of 

 the system, and surface tension is the intensity factor of the free surface energy. A 

 further analysis of the actual molecular forces involved may of course be made in 

 order to determine the mechanism whereby the surface tension is affected. 



Phagocytosis has frequently been considered as taking place in two stages (Kite 



