112 INFECTION AND IMMUNITY. 



is introduced into an animal, the incubation period 

 cannot be eliminated absolutely; and some of the 

 hemolytic toxins, as tetanolysin, can be added to 

 red blood cells in test-tubes in any desired quan- 

 tity without causing their immediate solution. 

 There is, therefore, something inherent in the 

 nature of these substances, or in the nature of the 

 action which they exert on the cells of the body, 

 which demands this latent period before an effect 

 becomes manifest. 



summary. It seems probable, therefore, that there are many 

 factors which contribute to the existence of the 

 incubation period, and that the factors which 

 determine its length in one instance may not be 

 identical with those which are found in another. 

 The time required for proliferation of the micro- 

 organism to an infective quantity probably is com- 

 mon to all infections. The infective quantity must 

 vary with different diseases, and with different 

 strains of the same micro-organism depending on 

 their virulence. A virulent strain has a shorter 

 incubation period than a less virulent. In some 

 instances a certain amount of time may be 

 required for the micro-organism to undergo an 

 increase in virulence sufficient to accomplish infec- 

 tion. Or, again, this time may be required for the 

 exhaustion (absorption or chemical binding) of 

 the protective substances to such a degree that fur- 

 ther proliferation and invasion take place with 

 greater rapidity, this latter step coinciding with 

 the onset of marked symptoms. The time required 

 for distribution of the poisons to vital organs would 

 seem to be of minor importance except in hydro- 

 phobia and tetanus, which utilize the peripheral 

 nerves as a route to the central nervous system. 



