sterilized in the autoclave for 15 minutes at 15 pounds pressure. 



(5) Beef brain agar: This preparation was made exactly as the beef 

 liver agar except that brain tissue was substituted for liver tissue. 



Thus prepared, the five media were compared by this procedure: Each 

 medium was tested with each experimental culture. We had, therefore, 

 six series of tests one for each strain. 0.05 c.c. of culture from the several 

 seed tubes were evenly distributed over the surfaces of agar slants of the 

 respective media. All the slants were formed by equal amounts of 

 medium. Readings were made after 1 8 hours incubation at 37 C. The 

 data tabulated below indicate the results obtained for one series. They 

 are representative since each series showed the same gradations in the 

 amounts of growth. 



The results are necessarily only comparatively quantitive. They 

 indicate, however, that the liver infusion furnished the optimum material 

 for maximum growth. This may be explained by the fact that the liver 

 normally contains a large store of glycogen, amino acids, various secre- 

 tions, etc., all probably valuable to bacterial- nutrition. 



(b) Concentration of agar-agar: The more concentrated the agar- 

 agar in a medium, the greater is the insurance against scratching the 

 surface of a solidified layer in a petri dish when removing growth with a 

 smooth spatula. But, on the other hand, too concentrated a preparation 

 will practically inhibit bacterial growth. It therefore, became necessary 

 to find that concentration which permitted the maximum development of 

 colonies and at the same time insured against the possible breaking of 

 the surface. Six series of tests were performed, one for each experimental 

 culture. The infusion subtrate was the same for all; the concentration of 

 the agar-agar was varied for each series. The results show definitely that 

 for the organisms used 2 per cent is the optimum concentration. 



