Staining Protozoa 195 



formed elements may be separated sufficiently for the indi- 

 vidual cells and organisms to be seen. 



Such a thin layer is usually easily obtained by the use of 

 a slide and cover-glass, and the careful preparation of a good 

 film. 



The slide and the cover-glass should be thoroughly cleansed 

 and freed from fat and grit and well polished. A compar- 

 atively small drop of blood let us say, for example is 

 placed upon the center of the slide and immediately covered 

 with the cover-glass. If the drop is not too large and the 

 glasses are clean, the weight of the cover-glass causes the 

 drop to spread, and capillary attraction completes the forma- 

 tion of a very thin film. The quantity of blood used should 

 not be sufficient to reach the edges of the cover-glass, else 

 sometimes the glass is pressed up instead of being drawn down 

 and moves about too freely. If the examination is to take 

 enough time to cause the drop to dry, a match-stick dipped in 

 thin vaselin and drawn about the edge of the cover will 

 prevent it. 



Such a film is usually best examined at or near the center, 

 where the formed elements are most widely separatedo 



The living protozoa in preparations of this kind may be 

 examined by ordinary illumination by transmitted light, or 

 with lateral illumination by means of the " dark-field il- 

 luminator." The latter serves better for the discovery 

 of the very small transparent organisms spirochaeta and 

 treponema and for the observation of the cilia and flagella. 



STAINING PROTOZOA. 



It is through the study of stained protozoa that we arrive 

 at most of our knowledge of their structural details. They 

 can be stained in blood or fluids upon a slide or in sections of 

 tissue. 



As in the case of the bacteria, it is first necessary to prepare 

 satisfactory spreads for the purpose. In order that the 

 description shall be as practical as possible, we will suppose 

 that the micro-organisms to be stained are in blood spiro- 

 chaeta, plasmodium, etc. 



As was pointed out above, the protozoa, under such circum- 

 stances, are distributed among or in cellular elements that 

 interfere with satisfactory observation unless precautions are 

 taken to separate them as widely as may be required. 



