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settle down far enough to come into contact with each other. As the tubes are ro- 

 tated both particles and cells describe circles in the fluid, the circumferences of which 

 are proportional to their relative speeds of settling, and return to their original posi- 

 tions at the end of each revolution. If both cells and particles settled with exactly 

 the same velocity, they could not readily come into contact and there would be very 

 little phagocytosis. 



If the number of cells remains constant during an experiment, as well as their 

 capacity for bacteria or particles, then the ingestion of the particles will follow an 

 exponential law. This law will not be strictly followed, however, if the cells become 

 clumped or if the particles are of unequal sizes or if they are aggregated into unequal 

 clumps. In such cases the faster-moving particles will have a better chance of over- 

 taking a cell and will be ingested first, thus changing the chances of collision and hence 

 the rate of the reaction for the suspension as a whole. Some problems of this type 

 have been considered by McKendrick' from a mathematical point of view.^ 



' McKendrick, A. G.: J. Physiol., 60, Proc. p. xxvii. 1925; Proc. London Math. Soc. (2d ser.), 

 p. 401. 1914; Science Progr., 8, 497. 1913-14. 



^ For further discussions of related subjects, cf. in this volume chap. Iviii, by Dr. Northrop, and 

 chap, xlii, by Dr. Falk. 



