BLOOD-SERUM. 95 



ratus of Koch be employed. The latter form is prefer- 

 able, as it is more easily managed. 



When solidification is complete, the tubes are to be 

 retained in the erect position, and, unless they are in- 

 tended for immediate use, must be prevented from dry- 

 ing. The superfluous ends of the cotton plugs should 

 be burned off, and the mouths of the tubes may then be 

 covered by sterilized rubber caps, or, as Ghriskey sug- 

 gests, they may be closed with sterilized corks pushed 

 in on top of the cotton plugs. Even with the greatest 

 care it not uncommonly happens that one or two of the 

 lot of tubes thus prepared and protected will become 

 contaminated. This is usually due to spores of moulds 

 that have fallen into the rubber caps or on the cotton 

 plugs during manipulation, and, finding no means of 

 outward growth, have thrown their hyphse downward 

 through the cotton into the tube, and their spores have 

 fallen on the surface of the serum and developed there. 



The foregoing is, in the main, the plan originally 

 recommended by Koch for the . preparation of this 

 medium. In recent times, however, particularly since 

 the study of diphtheria by the method of Loeffler has 

 become so general, and large quantities of serum tubes 

 were found to be necessary, a modification has been sug- 

 gested that has, in this country at least, almost entirely 

 supplanted the method by Koch. The popularity of 

 the Councilman-Mallory method is due to the following 

 facts : By it the serum is more quickly and easily pre- 

 pared ; rigid precautions against contamination during 

 collection of the serum are not so necessary, and the 

 resulting medium, while not transparent or even trans- 

 lucent (points aimed at in the original method), fully 

 meets all the requirements. 



