286 PRACTICAL DAIRY BACTERIOLOGY 



ferent samples, to balance the centrifugal machine on both sides. 

 Rotate in the machine at a rate of 2,000 to 3,000 per minute, 

 for 5 minutes. Remove the tubes from the machine and they 

 will be found to have a layer of cream at one end, and a small 

 slimy deposit at the other end next the cork. Holding the tube 

 with the cream end down, remove the cream with a platinum 

 loop, and pour out the milk gently. A slimy sediment will be 

 left attached to the cork. Carefuly remove the cork with its 

 adhering sediment, and smear over the surface of a glass slide, 

 with a drop of sterile water, to cover an area of exactly four 

 square centimeters. Special slides marked with wax pencils 

 into such areas are needed. This distribution of the sediment 

 must be carefully done in order that it shall be uniform. After 

 j:he thorough distribution, dry in gentle heat, or without heating, 

 and stain by flooding the surface with dilute 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 examined 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 in- 

 dicative 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 con- 

 siderable numbers of these leucocytes. To interpret the 

 meaning of what is found under the microscope in these prepa- 

 rations, see page 284. 



No. 18. Doane's Method of Determining Leucocytes. Place 

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

 2,000 per minute for four minutes. Carefully remove the cream 



