THE DETECTION OF PASTEURIZED MILK 157 



Even a cursory examination of the two smears shows a dis- 

 tinct and easily recognizable difference between the smear 

 from the heated milk and that from the raw milk. 



To examine the cells more closely, the oil immersion ob- 

 jective is used. The oil may be put directly onto the dried 

 smear, or the preparation may be mounted in Canada balsam 

 under a cover glass. 



The polymorphoiiuclear leucocytes in the raw milk, 

 under the high powers of the microscope, are practically all 

 colorless, but with some experience it is not difficult to 

 recognize them. Usually they are quite regular in outline 

 and large, i. e., about twelve microns in diameter. The nu- 

 clear material, if differentiated at all, is poorly defined and if 

 stained it is of a light greenish-blue color. Occasionally 

 there are leucocytes in raw milks, and in some more than 

 others, which take the stain. Whether or not these are 

 dead cells has not been determined. Even if quite deeply 

 stained the leucocytes in raw milk are distinctly different 

 from those in pasteurized milk, in that the nuclear 

 material is more spread out and less densely stained*. The 

 nuclei of the raw cells are usually integral with a definite 

 isthmus connecting the lobes. 



The leucocytes from properly pasteurized milks have their 

 nuclei deeply stained, and the different portions are frequently 

 rounded up into definite fragments so that the cells appear 

 to be polynuclear rather than polymorphoiiuclear. They aver- 

 age about seven microns in size. The depth of the stain and 

 the amount of shrinking vary somewhat with the degree of 

 heat applied. 



By way of summary it may be repeated that : 



The effect of heat on the leucocytes, so far as this test is 

 concerned, is twofold. It alters the shape and size of the cells, 

 and changes their staining reactions. The shape of the cell is 

 probably gradually changed as the degree of heat increases, and 

 the shrinking begins to appear at a lower temperature than that 

 used for pasteurization; but the "fixing" of the nuclear material, 

 which makes possible the absorption of the stain, seems to take 

 place definitely at practically the same temperature as that nee- 



