532 



H. J. SEARS AND LULU L. CASE 



It was always observed that when the experience just described 

 occurred it would be found that the 1-100 plates contained large 

 numbers of very small round colonies resembling streptococcus 

 colonies but differing from the latter in being whiter and more 

 opaque. These tiny colonies ranged in size from that of a pin- 

 head, to the merest points. The recognition of the latter as 

 colonies, even with the aid of the standard lens, was practically 

 impossible. It was only the gradation in size that indicated 

 that they were colonies and not specks in the medium. These 

 very minute colonies appeared on the 1-1000 plates only very 



rarely, and when they did they often were found on only one 

 of the duplicates. Their appearance always accompanied a wide 

 difference in the counts of the duplicate plates. Table 1 gives 

 the counts of fourteen samples selected from our records. 



The inconsistencies described seemed very mysterious at first 

 but on consideration it was seen that the explanation must lie 

 in the difference in composition between the nutrient medium in 

 the 1-100 plates and that in the 1-1000 plates. Inasmuch as 

 the same nutrient agar was used in each this difference could 

 only arise from the amount of milk introduced in the sample. 

 One one hundredth cubic centimeter of milk introduced into a 



