384 THEORETICAL RESPONSE OF CELLS TO CONTACT 



a large particle which made the same angle of contact with the cell. 

 Also, it is easier for a cell to spread around a small particle than around 

 a large one because the necessary mechanical deformation is less. 



Phagocytosis has been described repeatedly as taking place in two 

 stages (Kite and Wherry (3)), the actual ingestion being preceded by 

 a phase in which the object is merely stuck on the outside. The pre- 

 liminary stage is clearly a surface tension phenomenon. The fre- 

 quency of its occurrence is due to the fact that the surface of the cell 

 exposed to the plasma is thereby decreased.^ When objects seem to 

 be permanently stuck on the outside of a cell this may be a true 

 surface tension equilibrium or, more likely, it may be that the rigidity 

 of the structure of the interior of the cell prevents the further deforma- 

 tion necessary to reach a true surface tension equilibrium with the 

 object completely inside. 



In conclusion emphasis may be laid upon the significance, from 

 the point of view of surface tension of Tait's general proposition 

 which states that only unstable cells tend to be phagocytic. 



SUM]VL\RY. 



The theoretical behavior of a hypothetical fluid cell in contact with 

 flat and curved solid surfaces is discussed from the point of view of 

 surface tension. 



An equation is derived for calculating the equihbrium position of 

 the cell on a flat surface in terms of the surface tensions between the 

 ceU and the plasma, the plasma and the soHd surface, and the solid 

 surface and the ceU. It is shown that the same equilibrium is pre- 

 dicted from consideration of the contact angle between the cell and 

 the solid body. 



The relative surface energy has been calculated at various stages in 

 the ingestion of a solid particle by a fluid cell four times as large in 

 diameter, and it is thus shown that no particle will be ingested until 

 the surface tensions are such that the cell would spread to infinity on 

 a flat surface of the same substance. Here again the same equilib- 

 rium is predicted from considerations of the contact angle. 



^ This decrease is evidenced in Table I by the negative values of Ax when y 

 is 0.375 or less. 



