238 



Like Lloyd he removed the blood clot, but contrary to the 

 procedure of this author he laid stress on the fact that after 

 the addition of the blood the medium must only be heated 

 at 100° for a few minutes which is a point of considerable 

 importance. 



During the latter part of the pandemic and afterwards 

 these media with heated blood (in the various modifications) 

 have been more used than any of the other groups of media 

 mentioned. In America „chocolate agar" has principally been 

 used, in Germany, Levinthal's agar. 



Hundeshagen found that Pfeiffer's bacillus lived longer on 

 healed blood agar when the clot was not removed than on 

 Levinthal agar, and that a better growth was frequently ob- 

 tained on the first-named medium. These experiences, par- 

 ticularly the latter, may be regarded as general. We hardly 

 ever read that „chocolate agar" has given inconstant results, 

 whereas several authors report that Levinthal agar at times 

 could entirely fail or that it constantly gave a worse growth 

 than the discoverer claimed (Pribram (2), Gibson & Bowman, 

 Wyabd, Loewenhardt (2) and several others). Even Levin- 

 thal himself had a few mishaps with his medium 1 . He believes 

 that all the unsatisfactory results are due to too long heating 

 after the addition of the blood. On the other hand (Fildes, 

 Baker, & Thompson) it has been emphasized that only glass 

 wool ought to be used in filtering Levinthal agar, not cotton- 

 wool, gauze, or paper which may absorb the nutritive sub- 

 stances. Moreover filtration is not always necessary, the coa- 

 gulated blood often sinks to the bottom so rapidly that the 

 supernatant agar can be easily decanted off. 



It is probable that most of the untoward results with 

 Levinthal agar could be avoided by paying sufficient attention 

 to the two dangers mentioned, — too much heating, and con- 

 tact with finely divided cellulose. Whether this will be enough 

 in all cases can scarcely be decided at present. Even so it 

 cannot be denied that the removal of the clot and the consequent 

 diminished content in haemoglobin derivatives in Levinthal 

 agar is a defect in this medium, as it explains its sensitive- 

 ness to an uncautious technique of preparation, its limited 

 keeping qualities, and the comparatively poor resistance against 

 keeping of the cultures grown on it. In chocolate agar on 



