METHODS OF CULTURE AND OF EXAMINATION. 83 



upon the surface of the agar by means of a platinum 

 needle. 



(d) The preparation of liquid blood-serum for purposes 

 of culture requires a rather complex procedure. The blood 

 from a suitable animal, obtained after division of the carotid 

 artery (in slaughtering), is received into tall, sterilized glass 

 cylinders, and is permitted to stand upon ice for two days, 

 in order that the serum may completely separate from the 

 clot. The serum is then distributed among disinfected test- 

 tubes by means of, sterilized pipets, and placed for sev- 

 eral days in the thermostat at a temperature of 37 C. 

 (98.6 F.). By this means it is rendered certain whether 



Fig. 13. Flask to re- Fig. 14. Koch's apparatus for coagulating and 



ceive blood-serum. sterilizing blood-serum. 



all of the manipulations have been made without contami- 

 nation that is, whether the serum is still germ-free. In 

 this event the serum remains perfectly clear in the thermo- 

 stat. The tubes that become turbid are rejected. In order 

 to convert the liquid blood-serum into a solid culture- 

 medium, the tubes are placed in an oblique position in the 

 Papin steam-dish, and the serum is permitted rapidly to 

 solidify. After this the tubes are sterilized by exposure for 

 ten minutes on three successive days to a temperature of 

 100 C. (212 F.). 



For the preparation of serum-plates the liquid serum is 

 poured into sterilized Petri dishes (Fig. 23), and is then 

 solidified and sterilized in the same way as the serum in 



