42 GENERAL BACTERIOLOGY 



c. Make cover-glass preparations from the agar streaks and stain 

 with an aqueous solution of gentian violet or with Loeffler 's methylen 

 blue. 



d. Examine with the oil-immersion objective, and write the 

 names of the organisms in their proper places in the table below : 



e. Make sketches of each organism. 



/. Mount all preparations in balsam and hand them to instructor 

 for inspection. 



EXERCISE 23. STUDY OF CELL GROUPING. 



IMPRESSION PREPARATIONS. The exact relation of cell to cell as 

 they develop in the colony can frequently be determined best by 

 studying a "contact preparation" which is prepared as follows : 



a. Melt a gelatin tube and slope it, when solid make a streak 

 culture of B. mycoides and when growth has taken place dip the tube 

 in hot water to loosen gelatin, which is then slipped out of the tube. 



6. Lower gently a clean cover-glass over the surface. Apply a 

 slight pressure by tapping glass. Raise cover-glass by one edge, 

 taking care that natural arrangements of adherent bacteria are not 

 disturbed. 



c. Thoroughly air dry the same, then fix and stain in the ordinary 

 manner. 



