CONTAGION AND CONTAGIOUS DISEASES. 135 



Cohn, but by the ensemble of their morphological and physio- 

 logical properties, as exhibited in their life history (Crookshank). 



Again, Chauvin has pointed out that the Bacillus pyogenicus, 

 by modifying the basis on which it grows, may assume succes- 

 sively the character of a coccus, a long thread, or a spirillum. 

 This theory of pleomorphism is generally but not universally 

 accepted. 



It is also stated by some observers that bacteria are not 

 constant in their properties, and the experiments of N?egeli, 

 Davaine, Buchner, and Wernich seem to indicate that both the 

 morphological and physiological characters of microbes are 

 mutable, that changes in the nutrient medium may have effect 

 on their form and size, on their mode of multiplication, and on 

 their physiological and fermentative properties. 



Naegeli says that a given bacillus does not invariably produce 

 bacilli of the same structure, and does not always pass through 

 the same developmental stages. A bacterium which under given 

 conditions gives rise to a definite fermentation may lose this 

 property when cultivated under other conditions, and Buchner 

 states that the hay bacillus can be transformed into the Bacillus 

 anthracis. He says if hay bacilli are injected into the blood of 

 animals they do not give rise to anthrax. If, however, they 

 are bred for several generations in meat extract, and then in the 

 arterial blood of the rabbit, they acquire noxious properties, and 

 give rise to anthrax in mice after two to nine days' incubation ; 

 and conversely, if anthrax bacilli are properly cultivated, they 

 can be transformed into bacilli whose properties are identical 

 with those of hay bacilli. Without entering into the question 

 of the mutability both of form and physiological properties of 

 micro-bacteria, rejected by many observers, it is satisfactory to 

 know that the change in their power of virulence brought about 

 by cultivation and attenuation, which has already done so much 

 for the prevention and even cure of microbic diseases, will yet 

 do more in assisting in the suppression of many others. 



Period of Incichation. — The period of latency, or that interven- 

 ing between the reception of a disease-inducing microbe into the 

 economy and the manifestation of the effect, varies very consider- 

 ably. Some of the most virulent produce their deleterious effects 

 in a very short time, while others have a prolonged period of in- 

 cubation. For example, splenic fever and death may be induced 



