324 EGBERT S. BREED AND W. D. DOTTERRER 



than was anticipated and the dilutions were not carried far 

 enough to give assurance that the count was not affected by 

 overcrowding. In selecting individual plate counts which were 

 to be tabulated as satisfactory, those counts were chosen which 

 could be used in making an average without any individual 

 figure varying more than 20 per cent from the average. All 

 others were listed as discrepancies. For example, one sample 

 gave the following counts per plate, 1: 100 dilution 1944, 1472 

 and 1928 colonies; 1: 1000 dilution 484, 515 and 610 colonies; 

 1: 10000 dilution 43, 45, and 46 colonies. The counts of 

 484 and 515 from the 1: 1000 dilution were averaged with the 

 1: 10000 counts of 43, 45 and 46; and this average was taken 

 as the final count on the sample. The counts made on the 

 1: 100 plates were all listed as discrepancies because they are 

 more than 20 per cent lower than the average, and the count 

 of 610 from one of the 1: 1000 plates was also listed as a dis- 

 crepancy because it was more than 20 per cent higher than the 

 average. Occasionally all of the nine plates made from a sam- 

 ple could be included in the final average. 



Table I gives the number of plate counts made after five days 

 of incubation at 21°C., arranged in groups according to the 

 number of colonies which appeared on the plates. Four hun- 

 dred and thirty-nine of the 1435 plates had less than 10 colonies 

 per plate. Only 22.3 per cent of these checked within the 20 

 per cent limit. One hundred and eighty plates fell in the group 

 having more than 10 and less than 20 colonies per plate. Of these 

 53.9 per cent checked within the 20 per cent limit. Percent- 

 ages calculated for the groups of plates having 20 to 30, 30 to 

 50, 50 to 100, 100 to 200 and 200 to 400 colonies per plate were 

 more or less variable, showing that from 66.3 per cent to 93.2 

 per cent of the total number of plates agreed within the 20 per 

 cent Hmit. The best percentage of agreement is shown by the 

 group having more than 100 and less than 200 colonies per plate, 

 and the next highest by the group having between 50 and 100 

 colonies per plate. There were decidedly fewer plates giving 

 satisfactory results among those which had more than 400 colo- 

 nies per plate, the percentage of plates which checked within 

 20 per cent being 44.4. 



