V] METHODS OF INOCULATION 57 



The two methods — i.e., that of fractional culture and of 

 dilution — may be successfully combined in this way : from 

 the first or second new culture, estabHshed after the method 

 of fractional cultivation, in which after twenty-four or thirty- 

 six hours one species greatly predominates, draw out with a 

 large capillary pipette a droplet, and dilute this to a great 

 extent with the saline solution, as described above, and now 

 inoculate with a trace of this mixture a new culture-tube. 

 Or, if after twenty-four hours' incubation the microscope 

 reveals in this further culture more than one species, continue 

 the process of dilution and inoculation for a further genera- 

 tion. Thus it is possible to obtain cultures of only one 

 species, although the original fluid contained several species 

 of organisms. 



One of the best and universally adopted methods for 

 isolation is that of the plaie-cultivation introduced by Koch 

 in connection with the isolation of the choleraic comma 

 bacilli. A test-tube containing sterile nutritive gelatine as 

 above prepared is liquefied by gentle heat, best by being 

 kept in water of about 40° C., then the plug is lifted with 

 sterile forceps and the gelatine inoculated with a mere trace 

 of the bacterial mixture, either by means of the point of the 

 capillary pipette or of the heated and cooled point of a 

 platinum needle ; the plug is replaced and the gelatine 

 shaken so as to distribute uniformly the bacteria that had 

 been introduced. A shallow glass dish with flat bottom and 

 ground edge, and covered with a similar but slightly larger 

 dishji has previously been sterilised in the oven and then 

 allowed to cool ; the liquefied nutrient gelatine inoculated 

 with the trace of the bacterial material is then poured out 



^ These plate-dishes are known as Petri's dishes, but it ought to be 

 stated that several years before Petri I have described and figured this 

 dish — viz. , in fig. 9 (present fig. 11) of the third edition of this work. 



