194 PATHOGENIC BACTERIA. 



The blood agar-agar should be kept in the incubator a 

 day or two before use so as to insure perfect sterility. 



Blood-serum. — The great advantage possessed by this 

 medium is that it is itself a constituent of the body, and 

 hence offers opportunities for the development of the 

 parasitic forms of bacteria under the most natural con- 

 ditions possible. It is the most difficult of all the media 

 to prepare. The blood must be obtained from a slaughter- 

 house in an appropriate receptacle, the best things for the 

 purpose being tall narrow jars of about i liter capacity, 

 with a tightly-fitting lid. The jars are sterilized by heat 

 or by washing with alcohol and ether, are carefully dried, 

 closed, and carried to the slaughter-house where the blood 

 is to be obtained. As the blood flows from the severed 

 vessels of the animal the jars are filled one by one. It 

 seems advisable to allow the first blood to escape, as it is 

 likely to become contaminated from the hair. By waiting 

 until a coagulum forms upon the hair the danger of con- 

 tamination is obviated. The jars when full are allowed 

 to stand undisturbed until quite firm coagula form within 

 them. If these have any tendency to cling to the glass, 

 each one should be given a few violent twists, so as to 

 break away the fibrinous attachments. After this the 

 jars are carried to the laboratory and stood upon ice for 

 forty-eight hours, by which time the clots will have re- 

 tracted considerably, and a moderate amount of clear 

 serum can be removed by sterile pipettes and placed in 

 sterile tubes. If the serum obtained is red and clouded 

 from the presence of corpuscles, it may be pipetted into 

 sterile cylinders and allowed to sediment for twelve hours, 

 then repipetted into tubes. It is evident that such com- 

 plicated maneuvring will offer many possible chances of 

 infection ; hence the sterilization of the serum is of the 

 greatest importance. 



If it is desirable to use the serum as a liquid medium, 

 it is exposed to a temperature of 6o° C. for one hour 

 upon each of five consecutive days. If it is thought best 

 to coagulate the serum and make a solid culture-medium, 



