252 



(i). Specific agglutination and complement fixation. 

 Most of these points were investigated with 75 strains. 



15. Apart from a certain change in the intensity of growth 

 on long-continued cultivation all the strains of this organism 

 investigated entirely agreed with one another for all practical 

 purposes on each of the points enumerated, and also in the 

 absence of growth on ordinary agar in which they resemble 

 Pfeiffer's bacillus. 



16. In contrast to this great homogeneity in the species, 

 Bordet's bacillus (Bacillus tussis convulsivae), there exist in 

 the species Pfeiffer's bacillus (Bacillus influenzae), — in spite 

 of a large number of common characters, — a multiplicity 

 of individual differences between the strains. 



The most important of these are: 



(a). The division into indol-producers and non-indol-pro- 

 ducers. 



(b). The macroscopic appearance and consistence of the 

 culture. This applies to mass cultures on different media 

 and for single colonies. 



(c). Agglutination and complement fixation. 



(d). Minimum temperature for growth. 



16. The division into indol-producers and non-indol- 

 producers is very sharp. It has, with perhaps a few excep- 

 tions, proved to be independent of long-continued cultivation 

 and wide differences in the experimental conditions. 



Strong and weak indol-producers can be distinguished. The 

 latter constitute only a small minority. 



On suitable media the strong indol-producers form about 

 as much indol as Bacillus coli. 



17. On the basis of the macroscopic appearance and con- 

 sistence of the culture a classification of Pfeiffer's bacilli into 

 two not very sharply defined main groups, — „t y p i c a 1" and 

 „a typical" is made (table p. 145). This difference is par- 

 ticularly well brought out by the same experimental arrange- 

 ment as is employed to demonstrate the „symbiosis" phe- 

 nomenon. 



18. A definite method for showing the symbiosis phenome- 

 non has been evolved, based on the investigation of a series 

 of different factors of which the following may be specially 

 alluded to (19—21): 



