THE STAINING OF BACTERIA Ilf TISSUES. 107 



glass preparations of pure cultures^ as described in, 

 the preceding chapter. A minute quantity of blood is- 

 spread out quite evenly upon a clean bover-glass with 

 a needle, and is allowed to dry. It is even more im- 

 portant in this case than with preparations of pure 

 cultures to make the layer as thin as possible, for the 

 drop of blood contains an immense number of blood 

 coi'puscles, which, if they are too close together, or 

 even lying one upon the other in the dried up layer, 

 are certain to conceal the bacteria. The cover-glass 

 preparation is fixed in the ordinary way by passing 

 it, after it has been completely dried in the air, three 

 times through a flame. If the layer is somewhat 

 thicker, it is necessary to pass it through again, as by 

 this means the plasma and corpuscles are rendered 

 less capable of absorbing the stain, while the bacteria 

 are unaffected. It is a good plan to lay the fixed 

 blood preparation for a short time in 2 per cent, acetic 

 acid (generally about two minutes, but sometimes 

 longer) in order to dissolve out part of the plasma and 

 to extract the hsemoglobin from the blood corpuscles, 

 with the result that the preparation, with the exception 

 of the bacteria, becomes distinctly less inten'sely 

 stained. After this immersion in acetic acid, the 

 cover-glass is well rinsed with water, and then the 

 preparation is stained in the same manner as the dry 

 cover-glass preparations. 



Methylene blue is the most suitable reagent for 



