QUESTIONS. 75 



before introduction of the cotton plug. With solid media, 

 the fluid is allowed to solidify after inoculation, and after 

 solidification the paraffin oil is poured on the surface of the 

 medium. This effectually shuts out the oxygen, and as a rule 

 allows a luxuriant growth of anaerobic bacteria. 



2. Special apparatus, with oxygen replaced by hydrogen, is 

 also used for the cultivation of anaerobic bacteria. 



The Incubator and The Thermostat. 



For the purpose of growing bacteria it is often necessary 

 or desirable to obtain a constant temperature. The ordinary 

 body-temperature, 37 C., is the most favorable for the growth 

 of the pathogenic bacteria. Apparatus especially constructed 

 for maintaining a constant temperature are known as incuba- 

 tors or thermostats. They are generally made of double- 

 walled metal, and contain water, which by means of a gas-jet 

 at the bottom of the apparatus may be kept at a constant 

 temperature (Fig. 37). 



For regulating the gas-supply and to maintain the constant 

 temperature, instruments known as thermo-regulators are 

 used. A number of these are highly complicated, but for 

 ordinary work is recommended Reichert's or Dunham's mer- 

 curial thermo-regulator (Fig. 35). 



QUESTIONS. 



How do you inoculate a fluid culture-medium ? 



How do you inoculate potato cultures? 



In how many ways may gelatin tubes be inoculated ? 



What is meant by a stab-culture? By a slant culture? 



How are plate cultures made? 



Describe the method of making plate cultures according to Koch. How 

 are agar plate cultures made ? 



How are colonies on plates counted? 



Describe the method of making cultures in Petri dishes? 



How are Esmarch's culture-tubes made? 



Give a good method for the cultivation of anaerobic bacteria. What is an 

 incubator? 



What is a thermo-regulator ? 



