viii AGGLUTINATION OF B. PJESTIS 241 



The animal developed a bubo that suppurated and which 

 in the course of two weeks had completely healed up. 

 This rat was killed about five weeks after the first in- 

 jection, about two to three weeks after the last, and its 

 blood serum tested in corpore both on a guinea-pig and 

 on a rat. In each case 250 cubic millimetres of living 

 plague emulsion were mixed with 250 cubic millimetres 

 of blood serum. Both animals died of plague, as did the 

 control in each instance. 



It follows from these experiments that no conspicuous 

 amount of germicidal substance is produced in the blood 

 of the rat when it has been efficiently protected, whether 

 by Haffkine's prophylactic alone or by passing in addition 

 through a mild form of the plague. 



Plague therefore differs from some other infective 

 diseases (cholera, diphtheria, etc.) in the circumstance 

 that a previous immunisation of an animal against 

 plague does not necessarily create in the blood of this 

 animal an appreciable amount of germicidal substance 

 which may be in turn used for conferring either passive 

 immunity on a fresh animal or for neutralising a fatal 

 dose of living plague culture. It is necessary, however, 

 to remember that in all these experiments only relatively 

 small amounts of blood serum was used, and it is quite 

 possible that if huge doses of the serum, say several cubic 

 centimetres, had been injected, the result might have been 

 different. But for our purposes, viz. to see whether appreci- 

 able amounts of germicidal (specific) substances were 

 present in the blood, large doses of the serum were not to be 

 recommended. Recent research has shown that large doses 

 of blood serum of even normal blood contain substances 

 which act deleteriously on microbes. The above negative 



