126 Report of the Department of Bacteriology of the 



4. When cream is allowed to rise by gravity, practically all of 

 the cells adhere to the fat drops and are carried up into the cream 

 layer. Separators and centrifuges precipitate a variable propor- 

 tion of these cells and under some conditions may precipitate nearly 

 all of them. 



5. On account of this variable action of centrifuges, no method 

 of determining the number of the cells based on the examination 

 of sediments obtained in this way can give accurate quantitative 

 results. Such a method as the " smeared-sediment " method is 

 convenient to use for obtaining material for qualitative examination 

 but is of little value as a means of determining the number of cells 

 present. High cell counts obtained by this method indicate the 

 presence of large number of cells in the milk but low counts may 

 be due either to the failure of the centrifuge to precipitate or to a 

 low cell content of the milk. 



6. Strippings contain a larger number of cells than milk from the 

 earlier part of the milking. The variation in the number of cells in 

 the milk discharged from the four quarters of the udder is almost as 

 great as the variation in the number of cells discharged in the milk 

 of different cows. Individual cows show considerable variation in 

 the number of the cells from day to day. The cause or causes and 

 significance of these fluctuations are as yet unknown. 



A great deal of the confusion which has arisen in the discussion over 

 the nature of these cells and their hygienic significance has been due 

 to the assumption made by many investigators that the presence 

 of the cells under any conditions is a pathological phenomenon and 

 therefore undesirable. Many other contradictions have come about 

 because of the use of inaccurate methods of counting the cells and 

 in careless interpretation of results. The chief reason why histologists 

 are so far apart in their interpretation of the nature of the cells 

 is because they have not fully realized the extent of individual 

 variations and the consequent need of obtaining material from a 

 variety of animals whose previous history was known so far as cell 

 content of their milk was concerned. Histological material from 

 normal udders secured when they are secreting their maximum 

 quantity of milk is also very difficult to obtain, and few men have 

 ever studied such material. Moreover the interpretation of the 

 minute histological differences in the structure of the cells requires 

 careful technique and great care to avoid misinterpretations. 



