112 PATHOGENIC BACTERIA. 



After the agar-agar jelly solidifies its contraction causes 

 some water to collect at the lower part of the tube. This 

 should not be removed, as it keeps the material moist, 

 and also because it has a distinct influence upon the cha- 

 racter of the growth of the bacteria. 



Glycerin Agar-agar. For an unknown reason certain 

 of the bacteria which will not grow upon the agar-agar 

 as prepared above will do so if 3-7 per cent, of glycerin 

 be added. Among these is the tubercle bacillus, which, 

 not growing at all upon plain agar-agar, will grow well 

 when glycerin is added a fact discovered by Roux and 

 Nocard. The glycerin may also be added to gelatin or 

 any other medium. 



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 



