CHAPTER VIII. 



Technique of making plates— Esmarch tubes, Petri plates, etc. 



Plates. — The plate method can be practised with 

 both agar-agar and gelatin. It cannot be practised 

 with blood-serum, because the serum, when once solidi- 

 tied, cannot be again liquefied. 



Plates are usually referred to as " a set." This term 

 implies three individual plates each representing the 

 mixture of organisms in a higher stage of dilution. The 

 first plate is known usually as "the original," or "plate 

 1," the first dilution from this as " plate 2," and the 

 second as " plate 3." 



In the preparation of a set of plates the following are 

 the steps to be observed : 



Three tubes, each containing from 7 to 9 c.c. of gela- 

 tin or agar-agar, are placed in the water-bath until the 

 medium has become liquid. If agar-agar is employed, 

 this is" accomplished at the boiling-point of water ; if 

 gelatin is used, a much lower temperature suffices (35°- 

 40° C). When liquefaction is complete the temperature 

 of the water, in the case of agar-agar, must be reduced 

 to 41°-42° C, at which temperature the agar-agar re- 

 mains liquid and the organisms may be introduced into 

 it without fear of destroying their vitality. The medium 

 being now liquid and of the proper temperature, a very 

 small portion of the mixture of organisms to be studied 

 is taken up with a sterilized, looped platinum wire, an 



