INFLUENZA. 283 



developing in the control-tubes not previously treated with 

 blood. The inoculated test-tubes are placed in the thermostat. 

 After the lapse of twenty-four hours the influenza-colonies will 

 become visible in the blood-agar tubes as densely packed, 

 watery drops, whereas the control-tubes will be either sterile 

 or present isolated colonies of streptococci, diplococci, or other 

 bacteria, which were present in the infecting material in addi- 

 tion to the influenza-bacilli. 



The water-like drops of influenza-colonies are usually so small 

 that they are only visible with the aid of a lens. They exhibit 

 little tendency to become confluent. Should they be especially 

 numerous and close together, they coalesce to form larger drops 

 with curved margins, but these permit a recognition of the aggre- 

 gation of individual colonies. If the colonies are isolated and 

 widely separated from one another, they may grow to the size 

 of a pinhead, but in this case also they retain a vitreous trans- 

 parency. The water of condensation of influenza-cultures gen- 

 erally remains clear. When mixed with blood derived from the 

 oblique surface of the culture, delicate white flocculi form in it. 



In bouillon mixed with blood and spread in a thin layer the 

 influenza-bacillus thrives quite abundantly. 



The plate-procedure is greatly to be recommended for the iso- 

 lation of influenza-bacilli and for diagnostic purposes, if some 

 blood is added to the liquefied agar before inoculation, or 

 if agar is permitted to solidify in Petri dishes and blood is 

 added, and the diluted sputum, etc., is smeared upon the sur- 

 face in several streaks. The colonies present the same appear- 

 ance as those in agar-tubes. 



The tempe-rature- optimum for the cultivation of influenza- 

 bacilli is that of the body. The upper limit for growth is 42 

 C. (107.6 F.), the lower between 26 C. (78.8 F.) and 27 

 C. (80.6 F. ). The bacilli do not grow at room-temperature. 



Oxygen is always necessary for the development of the influ- 

 enza-bacilli ; they do not grow in an atmosphere of hydrogen 

 or of carbon monoxid, even in the presence of blood. 



Pfeiffer undertook to determine what constituent of the blood 

 the influenza-bacilli require for their development. On making 

 transfers from blood-serum or blood-fibrin to agar-tubes no 

 growth took place. Red blood-corpuscles were invariably 

 necessary, and particularly the hemoglobin contained in them, 

 as was later shown, was the active substance. Hemoglobin- 

 agar (p. 82) is equally adapted with blood-agar for the cultiva- 

 tion of influenza-bacilli. Pfeiffer endeavored to associate this 

 indispensability of hemoglobin for the growth of influenza- 

 bacilli at first with its relations to oxygen, with its faculty of 

 acting as an oxygen-carrier. He succeeded, however, in ob- 

 taining growth upon an agar-layer in the presence of carbon- 



