PLATE CULTIVATIONS 



79 



diameter, known as Petri dishes (Fig. 14), are almost 

 always employed. They are previously sterilised in the 

 hot-air steriliser in suitable iron or copper boxes holding 

 a dozen or so ; the melted gelatin having been poured in, 

 the dish is tilted to diffuse the gelatin over the bottom of 

 the dish, placed on a level surface for the gelatin to set, 

 and then stored in the cool incubator. The plates are 

 examined daily, with a hand lens if neces- 

 sary, or with a low power of the micro- 

 scope, the dish being turned bottom 

 upwards on the stage of the microscope 

 for this purpose. When the colonies have 

 developed, inoculations can be made from 

 them by means of a platinum needle on 

 to tubes of sterile media. The colonies, 

 having arisen from single organisms, are 

 pure, and the resulting sub-cultures are 

 therefore also pure (it sometimes happens 

 that the colonies are mixed owing to two 

 or more organisms being dose together). 

 Different species of organisms usually form 

 colonies having different appearances, so 

 that the colonies are an aid in diagnosis and enable the 

 various species to be picked out from a mixture. The colonies 

 in gelatin are as a rule much more distinctive than those in 

 agar. Whereas the plate cultivation prepared from tube 

 No. 1 is generally too crowded, plates 2 or 3, or both, can 

 be made use of, and it is apparent that, to make certain 

 of isolating all the organisms from a mixture, several 

 sets of plates should be prepared. Flat bottles (Fig. 15) 

 may likewise be used for plate culturing, and are also very 

 useful for growing organisms in bulk for the examination 

 of the constituents and actions of the bacterial cells. 



Golding has devised flat wedge-shaped flasks (having 

 sides at an appropriate angle) for plate-culturing, and 



Fio. 15. "Plate 1 

 bottle. 



