BY R. GREIG-SMITH. 163 



limitation may be, it is apparently something which is slow- 

 growing, for an infection with 5% of I'aw soil only begins to 

 show a limiting action upon the fortieth day. It is also some- 

 thing, like the larger protozoa, which can be retained by cotton- 

 wool filters. These characters would apply to the Actinomycetes 

 which have the property of growing slowly in felted masses 

 capable of being retained by cotton- wool with the particles of 

 soil and the larger protozoa. It appeared possible, therefore, 

 that, in the Actinomycetes, and possibly in an excessive growth 

 of them, we might find the limiting factor for which bacteriolo- 

 gists have been seeking. 



Russell and Hutchinson's researches would appear to negative 

 this possibility, for they found that the Actinomycetes were less 

 affected than other forms by toluening; in fact, that immediately 

 after toluening they were the predominating species of micro- 

 organisms, and they retained this position for nine days. Of 

 the two forms of Actinomyces, the white and the brown, tlie 

 latter predominates in raw soil, while the former is conspicuous 

 in toluened soil. Owing to the presence of the white form in 

 toluened soil, in which the limiting factor had been destroyed, 

 Russell and Hutchinson naturally did not test the Actinomycetes 

 to see if they could functionate as the limiting factor. Their 

 apparent toxicity upon agar plates, however, was so conspicuous, 

 that I considered a test was necessary to make tlieir position 

 definite. 



Among the species which appeared to be most toxic, three 

 were picked out as being the most promising. Two of these, 

 Nos. 10 and 14, were much the same, and grew as dirty white 

 crusts on nutrient agar; 14 was a trifle darker than 10. Both 

 adhered firmly to the agar surface. The third, No.ll, formed a 

 loose, yellow growth, part of which came away easily from the 

 agar; microscopically, it appeared as a rod with a tendency to 

 grow in threads At first, it produced a yellow pigment, especi- 

 ally upon Lipman-Brown agar, which diftused through the 

 medium. Cultural experiments showed that ordinary nutrient 

 agar was the best for obtaining growths of these species: Huid 

 media were quite unsuitable for growing them in any quantity. 



