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 direetly onto the dried 
smear, or the preparation may be mounted in Canada balsam 
under a cover glass. 
The polymorphonuclear 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 distinetly 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 polymorphonuclear. 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 nec- 
