LABORATORY WORK 295 



,l>e carefully done in order that it should be uniform. After the 

 thorough distribution, dry in gentle heat, or without heating, and 

 stain by flooding the surface with dilute solution of methylene blue 

 for a short time, and then wash the stain away. After again drying, 

 the material may be mounted under cover-glass, or it may be exam- 

 ined immediately with an immersion lens without a cover-glass. 

 There will be found on the slide a considerable number of stained 

 bacteria, usually showing a variety of forms, and also a varying 

 number of large cells, most of which have deeply stained nuclei, but 

 unstained bodies. These latter are leucocytes, and represent cells of 

 the character of the white blood cells from the cow, which have 

 found their way into the milk. In some cases, leucocytes represent 

 pus cells and are indicative of inflammatory changes in the animal's 

 udder. Where the numbers are small, however, they give no such 

 indication, for the normal milk of healthy udders will usually show 

 considerable numbers of these leucocytes. To interpret the meaning 

 of what is found under the microscope in these preparations, see 

 section 4. 



4. Doane's Method of. Determining Leucocytes, — Place 

 10 c.c. of milk in the tubes of a centrifugal and rotate at about 

 3,ooo per minute for four minutes. Carefully remove the cream 

 from the surface with a bit of cotton on the end of a rod, being 

 careful not to leave any of the cream in the tube. By means of a 

 small siphon remove the milk, keeping the tip of the tube just below 

 the surface so as to avoid disturbing the sediment. Siphon away 

 the milk to within about one-eighth of an inch of the sediment. Add 

 two drops of a saturated alcoholic solution of methylene blue to the 

 tube, and, after thoroughly mixing, place the tube in boiling water 

 for two minutes to aid the staining. Add enough water to bring 

 the total bulk either to one or two c.c, according to the amount of 

 sediment. This will give a blue mass of stained sediments. To 

 count their number an ordinary blood counter is used. This will 

 have a counting chamber marked off into squares and holding ex- 



