108 Animal Microloyy 



C. Hanging-Drop Preparations 



1. A slide with a concave center is used (Fig. 40). With a fine-pointed 

 brush paint a narrow strip of vaselin around the margin of the concavity. 

 The vaselin makes the cover-glass stick to the slide and also prevents 

 evaporation. 



2. Place a small drop of the fluid containing bacteria in the center of 

 the cover-glass. If the bacteria to be examined are on a solid medium 



the "drop" should be made by 

 -J ' .3r:e:b*1 mixing a small portion of the 



growth with a drop of bouillon, 

 FIG. 40.-Culture Slide. 



normal saline, or serum. Place 



the cover-glass, drop downward, over the depression in the slide and 

 press it down well into the vaselin. 



3. Use only a small opening in the diaphragm when examining the 

 bacteria, in order to get as much contrast by refraction as possible. 

 Focus first with a medium-power dry objective on the edge of the drop, 

 then employ the oil immersion. Such unstained organisms are fre- 

 quently difficult to find and there is great danger of breaking the cover- 

 glass with the objective. 



Hanging-drop preparations are used mainly in determining the 

 motility of bacteria, or in the study of spore formation. For the latter 

 purpose, the slide and cover-glass must be carefully sterilized and the 

 sealing with vaselin complete. The preparation may then be placed on 

 a warm stage or in an incubator and examined from time to time. 



MEMORANDA 



1. The Main Points to Be Observed in the Microscopical Examination of 

 Bacteria are as follows: (1) form of the individual, whether spherical 

 (coccus), spiral (spirillum), or rodlike (bacillus) with end square, pointed, 

 or rounded; (2) uniformity in size; (3) the arrangements of individuals 

 whether single (micrococci, etc.), in pairs (e. g., diplococci), in chains 

 (e. g., streptococci), groups of four (e. g., tetracocci), cubical groups of 

 eight or more (sarcinae), or small grape-like bunches of various-sized 

 cocci (staphylococci); (4) presence or absence of cell-wall, gelatinous 

 capsule, etc.; (5) motility in living forms (do not confuse with Bro \vniau 

 movement); (6) reaction to stains; (7) presence of spores which are rec- 

 ognizable as bright, highly refractive rounded bodies. 



2. Material for the Demonstration of Bacteria (coccus, bacillus, spirillum, 

 and beggiatoa forms) will be found in abundance in foul water, espe- 

 cially when contaminated with sewage. By scraping the inside of the 

 cheek such forms as Leptothrix may often be found. Make a cover- 

 glass preparation; kill and fix in the flame in the ordinary way; stain in 



