44 MASS. EXPERIMENT STATION BULLETIN 319 



Strain Xll-a was recovered, and the growth resembled that of the original variant 

 type. (See Figures 17 and 75.) 



Lot XI. The chicks in this group were inoculated with variant VI Il-a. A dose 

 of 0.4 cc. was administered daily for the first seven days and 0.5 cc. for the next 

 seven days. No apparent symptoms were observed. Necropsies were conducted 

 one week after the last injection. Two chicks yielded 5. pullorum. The organism 

 appeared typical (normal) and in no way resembled the variant originally inocu- 

 lated. In one instance the organism was isolated from the spleen and necrotic 

 tissue at the site of inoculation, while in the other it was isolated only from the 

 necrotic tissue. Lesions in the remaining chicks were of no significance. The 

 recovered organism underwent characteristic agglutination with positive pullorum 

 serum, while with negative serum no agglutination or settling was seen. The 

 serum of the chicks was tested with the homologous antigen and with typical 

 pullorum antigen. The homologous antigen (VHI-a) revealed a tendency to 

 settle out in the first few dilutions, while no agglutination or settling was observed 

 with the pullorum antigen. 



Table 14.— Comparison of results of inoculation with parent and 



VARIANT STRAINS OF S. pullorum. 



Table 14 gives a comparison of the results obtained with parent strains and their 

 variants, when inoculated into chicks. Among the 120 chicks inoculated with the 

 parent strains, 48, or 40 per cent, died. Among the 130 inoculated with the 

 variants, 5, or 3.85 per cent, died. 5. pullorum was isolated from 90, or 75 per cent, 

 of the chicks inoculated with the parent strains as compared with 21, or 16.15 

 per cent, of those injected with the variants. The differences between the two 

 groups in the percentage of mortality and positive isolations are very significant. 



Beyond a doubt, some of the variants studied here definitely appear less path- 

 ogenic for chicks than their parent strains. Variants Ill-a and VHI-a, which were 

 administered in frequent doses without producing symptoms, must have under- 

 gone a profound change in their structure, since their parent strains were among 

 the most pathogenic of the strains studied. Relative to the two positive isolations 

 from chicks in Lot XI, one may question whether some of the cell variants re- 

 verted to the normal type, or whether the strain was not a pure variant, and 

 contained a few normal cells which, upon repeated injection, gradually established 

 themselves as a so-called typical strain. The author is inclined to subscribe to the 

 latter view, although this is still a matter of speculation, since it is impossible to 

 follow the cell progeny of a mass culture. 



It appears that repeated injection of a variant is a means of determining the 

 purity of such a type. As is shown in Table 13, types Ill-a and Vlll-a failed to 

 establish themselves in chicks after five inoculations, but after 14 inoculations 

 of Strain VHI-a positive results were obtained. The fact that this variant was 

 inoculated into the natural host may also account in part for the positive results. 

 Thus, before designating a strain as a pure variant, it would seem advisable to 



