308 BACTfcRIA. 



developed is as yet small ; inoculating this into an animal it 

 would be found that death would take place at the end of a 

 certain period, and that, on examination at the point of 

 inoculation, numerous bacilli, evidently the result of vege- 

 tative growth of the micro-organism within the tissues at 

 this point, might be demonstrated ; whilst on the other hand, 

 if an old cultivation (in which the organisms are weak, and 

 in which many involution forms are present, but in which 

 there is a large quantity of poison which has been developed 

 by the micro-organisms during their period of activity, and 

 which from standing has become diffused into the liquid) be 

 inoculated, death will be produced very rapidly, but no 

 organisms can be found at the seat of inoculation ; there is 

 set up, in fact, a true toxic condition. If the residues left on 

 the filter from the above cultures were inoculated, it would be 

 found that in the case of the young culture, death of the 

 animal would take place at very much the same period as 

 when the unfiltered culture was inoculated ; whilst in the 

 case of the old culture the period at which death takes place 

 is very much delayed ; in the one case, the organisms 

 are active, although deprived of the poison which they form ; 

 they can live in the tissues, and can produce fresh poison ; 

 whilst the older organisms and involution forms, no longer 

 able to develop in the tissues, and deprived by the filtration of 

 the poison they produced whilst they were active, sometimes 

 do not cause the animal to succumb even at a late date. 



This virulence, then, is associated with the power of a microbe to develop 

 in the body of a living animal, a power which may be considerably increased 

 by passage of the organism through a series of suceptible animals, the 

 bacillus acquiring a more and more parasitic habit in each successive host. 

 Researches on micro-organisms are of no value to medicine unless they 

 throw light directly or indirectly on the cause of disease, and so enable 

 the physician to combat its advance. At first sight it would seem that in 

 the case of the diphtheria bacillus there is, on account of the extreme activity 

 of the poison, little hope of rendering the tissues of an animal resistant to 

 its action, as even very minute doses produce marked poisonous effects. On 

 the other hand, however, we have, from the nature and position of the 

 development of the poison, indications as to treatment and also as to 

 prevention of the disease. 



The indications as to prevention are of course similar to 

 those for other micro-organismal diseases ; if we know the 

 natural history of the diphtheria bacillus, we know at what 

 point it is most vulnerable, the conditions favourable for its 



