160 UNIVERSITY OF WISCONSIN STUDIES 



Frequently there is a tinge of blue to them so that there is 

 just a suggestion of the nuclear structure of the cell visible. 

 A few cells even in perfectly fresh milk may have their nuclei 

 stained but less densely and compactly than in pasteurized 

 milk. In the pasteurized milk, on the other hand, the nuclei 

 are definitely stained, usually dark blue, while the background 

 is lighter. The difference is striking and perfectly charac- 

 teristic. 



Outline of leucocytes. The outline of the cells in raw 

 milk is irregular. This is probably due to protrusions or 

 pseuclopodia although certain authors have hesitated to accept 

 these cells as leucocytes because they do not show evidence of 

 amoeboid movement. The irregular outlines, as shown in the 

 figures of raw milk, are noticeable, however, especially in 

 Figures 4 and 13. 



In the heated milk the outline of the cells usually appears 

 more regular. Under the influence of heat the cells have 

 rounded up. There is no constant difference between these 

 cells under the two conditions, however, and some observers 

 have even regarded the heated cells as the more irregular. 

 The irregularity of the heated cells may, however, be due to 

 quite a different cause, namely, shrinkage. 



Nuclear fragments. Only the polymorphonucleated cells 

 are of value in this test. In a raw cell it is usual for the nu- 

 cleus to be single but variously shaped (or polymorpho), while 

 in the heated milks there is a pronounced tendency for the 

 nuclei to become separated and thus the cells become poly- 

 nucleated. The nucleus or its fragments are much more 

 compact and demarkated in the heated than in the raw samples. 



Size of the leucocytes. As already pointed out, the heat 

 of pasteurization not only affects the staining reaction of 

 the leucocytes but it produces a profound change in their size. 

 In raw milk these cells are frequently 10 or 12 microns in 

 diameter and nearly always above 7.5 microns. In properly 

 pasteurized milks they are very much smaller, usually less 

 than 7.5 microns. 



This marked reduction in size is no doubt due to the heat 

 of pasteurization which shrinks the cells. The decrease in 

 size is a progressive one, depending upon the amount of heat 



