VIRUS PARTICLES AND THEIR FUNCTIONAL ACTIVITY 347 



virus particles to infect a single susceptible cell, the lesion comit might be 

 expected to increase more rapidly than linearly with increasing virus con- 

 centration, in contrast to the usual experimental findings (Luria, 1953). 

 Fm'thermore, if virus particles are randomly distributed in dilute suspension, 

 the frequency distribution of such particles in samples taken from the 

 suspension should follow Poisson's law of small numbers. This, in turn, 

 should be reflected by a similar distribution of lesions upon inoculation of 

 samples, provided the infective particles act independently. And, indeed, 

 such a distribution of plaques has been found by Ellis and Delbriick (1939) 

 upon plating aliquots of a dilute suspension of coliphage. 



Although it appears that under appropriate conditions (i.e., at low 

 multiplicity of infection) lesions are produced by single virus particles, this 

 gives no information about the total number of virus particles present in the 

 inoculum. The latter number can only be ascertained with assurance from 

 direct particle comits by electron microscopy (Section III, A), from which 

 one can then estimate the "efiiciency of plating" or the probability that a 

 virus particle will infect. 



There has been some question recently of the assertion that the distribution 

 of pock counts on the chorioallantoic membrane of chick embryos follows the 

 Poisson series. If the pock comits truly reflect random distribution of virus 

 particles in suspension prior to inoculation, the variance, V, of the mean 

 count is expected on a Poisson distribution to equal the mean count, x, from 

 which the expected coefficient of variation {100^/ V I x) is readily calculated. 

 Westwood et al. (1957), upon comparing the theoretical and observed 

 coefficients of variation of vaccinia virus pock counts on chorioallantoic 

 membranes, usually found the latter to exceed the former, indicating that the 

 degree of scatter of experimental counts was in excess of that expected, 

 although by careful control of experimental conditions the distribution of 

 counts approached that predicted by the Poisson equation. Kaplan and 

 Belyavin (1957), using the same virus assay system, found not only that the 

 variances of mean pock counts exceeded expectation (on the assumption 

 that the distribution was Poissonian) but also that they "wandered" exces- 

 sively, makmg impossible valid estimates of coefficients of variation. In a 

 detailed statistical study of the variability of pock count data obtamed by 

 others, Armitage (1957) also concluded that the variance was considerably 

 greater than that expected if the count distributions were of the Poisson 

 form. By way of a possible explanation of the excessive deviation of 

 pock counts from theoretical, the hypothesis frequently offered is that the 

 chorioallantoic membranes are heterogeneous with respect to virus suscep- 

 tibility. Thus a secondary distribution of host susceptibility is super- 

 imposed upon the random sampling distribution of virus particles in 

 suspension. 



