MILK ID/ 



until an exact number of sides of the squares spans a diameter 

 of the field of vision. 



The number of cellular elements per cubic mm. of 



milk= , where j/ = the average number per field of vision, 



^=the number of squares which just spans the diameter, d is 

 determined once for all by marking the microscope draw tube so 

 that only 20 fields have to be counted, and the figures substituted 

 in the formula. 



B. Examination of the stained centrifngalised deposit. To 

 obtain comparable results the sediment from a definite amount 

 of milk should be examined after centrifugalisation for a definite 

 period. Ten c.c. of milk centrifugalised for 10 minutes is con- 

 venient. Part of the deposit is spread thinly but uniformly 

 over a cover-slip, dried in air, fixed in the flame, or preferably by 

 soaking in a mixture of equal parts alcohol and ether for one 

 minute, stained by methylene blue and mounted in balsam. 



The preparation may be utilised to gain an idea of the 

 general bacterial content, whether streptococci are present, and if 

 so in what numbers and whether intracellular, while, if considered 

 necessary, a differential count may be made of the cellular 

 elements present. For this purpose not less than 200 should 

 be enumerated. With care a rough but valuable estimate can 

 be obtained from this examination as to the probable number 

 of bacteria in the sample. 



C. Determination of the number of streptococci. To estimate 

 the number of streptococci in milk the method recommended as 

 the simplest and most reliable is to add diluted fractions of the 

 milk, ro, O'l, O'OI, O'OOI c.c. etc., to tubes of glucose neutral 

 red broth. Ordinary broth will do, but the neutral red broth 

 is preferable and gives better results. The tubes are incubated 

 for two days at 37 C. and then examined, in hanging drop 

 preparation, for streptococcus chains. The deposit should be 

 selected for examination, and several hanging drop preparations 

 made. A positive result should only be recorded when quite 

 definite chains of cocci are detected, or, in doubtful cases, when 

 stained preparations show such definite chains. 



