227 



organism becomes visible to the naked eye as a " colony." 

 This is the principle upon which the method of plate cul- 

 tivation is based and its practice enables the bacteri- 

 ologist to study the particular manner of development 

 affected by each species of microbe when growing 

 (a) unrestricted upon the sur- 

 face of the medium, (b) in the 

 depths of the medium. The 

 method itself is as follows : 



Apparatus Required. 



1. Tripod levelling stand. 



2. Large shallow glass dish, with a 

 square sheet of plate glass to cover it. 



3. Spirit level. 



4. Case of sterile Petri dishes. 



5. Tubes of sterile nutrient media, 

 gelatine (or agar) previously liquefied 

 by heating in the water-bath and 

 cooled to 42 C., otherwise the heat 

 of the medium would destroy many, 

 if not all, of the bacteria introduced. 



6. Tube of cultivation to be planted 

 from. 



7. Platinum loop. 



8. Bunsen burner. 



9. Grease pencil. 



Method of " Pouring" Plates. water ba f fo . r eltin s * 



agar and gelatine previous to 



Plapp tVip alfl d rl* Vi rm tVip slanting them; or to making 

 * "shake cultures or pouring 



levelling tripod (Figs. 122, 123); plates, 

 if gelatine plates are to be poured fill the dish with 

 ice water gelatine solidifies so slowly that it is 

 necessary to hasten the process ; if agar is to be used 

 fill with water at 50 C. agar sets almost immediately 

 at the room temperature and by slightly retarding the 

 process lumpiness is avoided ; cover the dish with the 

 square sheet of glass. 



2. Place the spirit level on the sheet of glass and by 

 means of the levelling screws adjust the surface of the 

 glass to the horizontal. 



FIG. 121. Handy form of 



