STUDY OF THE PROBLEMS OF IMMUNITY 333 



saprophytes of the mouth, and plate cultures are 

 rapidly overgrown by them to the destruction of the 

 more delicate pneumococcus. But inoculate some of 

 the sputum under the skin of a mouse and three or 

 four days later the pneumococcus will have entered 

 the blood stream (leaving the saprophytes at the seat 

 of inoculation) and killed the animal. Cultivations 

 made at the post-mortem (see page 398) from the 

 mouse's heart blood will yield a pure growth of the 

 pneumococcus . 



C. Identification of Pathogenetic Bacteria. 



The resemblances, morphological and cultural, exist- 

 ing between certain pathogenetic bacteria are in some 

 cases so great as to completely overwhelm the differ- 

 ences; again the same bacterium may under varying 

 conditions assume appearances so different from those 

 regarded as typical or normal as to throw doubt on its 

 identity. In each case a simple inoculation experiment 

 may decide the point at once. As a concrete example 

 may be instanced an autopsy on an animal dead from 

 an unknown infection. Cultivations from the heart 

 blood gave a pure growth of a typical (capsulated) 

 pneumococcus. Cultivations from the liver gave a 

 pure growth of what appeared to be a typical (non- 

 capsulated) Streptococcus pyogenes longus. The latter 

 inoculated into a rabbit caused the death of the animal 

 from pneumococcic septicaemia, and cultures from the 

 rabbit's blood gave a pure growth of a typical (capsu- 

 lated) pneumococcus. 



D. Study of the Problems of Immunity. 



It is only by a careful and elaborate study of the 

 behaviour of the animal cell and the body fluids vis-a- 

 vis with the infecting bacterium that it becomes pos- 

 sible to throw light upon the complex problem whereby 

 the cell opposes successful resistance to the diffusion 

 of the invading microbe, or succeeds in driving out 



