WALLACE O. FENN 581 



was then allowed to run under the cover-slip and the edges were 

 sealed with paraffin to prevent evaporation. The slide was at once 

 placed on a warm stage at 37°C. and frequent counts were made of 

 the number of free particles of quartz and carbon over a given area. 

 The areas were measured by a disc micrometer in an ocular ruled 

 in squares. It was customary to count sixteen such areas for each 

 point. 



The advantages of this method are : 



1. The chance of collision depends merely upon the size which the 

 particle offers as a target for the cells; i.e., (C + P) which is so small 

 a factor that it is almost neghgible.^ 



2. Agglutination of particles is impossible. 



3. Both quartz and carbon particles can be incubated together in 

 the same film so that the number of active cells is identical at any 

 one time. 



The only disadvantage of the method is that the cells are injured 

 by contact with the glass slide, so that the time curves do not follow 

 the law for a monomolecular reaction as they otherwise would. Thus 

 the cells are observed during the course of the reaction to spread out 

 on the glass, becoming vacuolated and transparent and ceasing their 

 activities. 



Phagocytosis with the film method presents an interesting picture, 

 as shown in the photographs (Figs. 8 and 9). Nearly all the cells 

 are in active motion. The activities of one cell were recorded for 

 3 minutes during which time it refused one quartz particle and one 

 carbon particle, and ingested three carbon particles, nearly reaching 

 a fourth. No evidence was obtained that cells can sense particles 

 from a distance, as reported by Commandon (6) for leucocytes ingesting 

 starch grains, and by Schaeffer (7) for amebae in proximity to carbon 

 and glass. All the meetings seem to occur purely by chance. Sudden 

 movements of leucocytes for a distance 2 or 3 times their own length 

 have frequently attracted attention, as if the cell had been under 



^ C = diameter of cell, P = diameter of particle. This is merely the target 

 factor from the formula for the chances of collision as used in the suspension 

 method. In this instance, however, the chance of collision is (C -\- P), not 

 (C + PY, because motion is confined to two dimensions. 



