COLLODION SACS 121 



rule can be given as to the duration of life of cultures on 

 artificial media. Most organisms will retain their vitality 

 for at least three or four weeks without being transferred 

 to a fresh soil, some for many months ; a few must be 

 sub- cultured every week, or they will die out ; while 

 there are still a small number which have so far rarely or 

 never been cultivated. On the whole, organisms retain 

 their vitality best on gelatin. 



For an organism to retain its virulence it is, as a rule, 

 necessary to pass it through a susceptible animal at longer 

 or shorter intervals, and to enhance the virulence recourse 

 must be had to a succession of passages through susceptible 

 and then less susceptible animals. In this way the viru- 

 lence of organisms has been increased to a point far greater 

 than is ever met with naturally, as in the case of the 

 Streptococcus pyogenes. If an organism retains its virulence 

 even slightly, it is generally possible, by employing large 

 doses, to enhance this by passage through a susceptible 

 animal. Another method may also be adopted, namely, 

 to inject along with it some other pathogenic form, such 

 as the Streptococcus pyogenes ; the combination will kill 

 the animal, and the slightly virulent organism can be 

 recovered and will be found to have increased in virulence. 

 A third method is to inject the organism into a susceptible 

 animal together with a lethal dose of toxin obtained from 

 a virulent form of the same species, or with some substance, 

 such as lactic acid, which lowers the vitality of the tissues. 

 The slightly virulent organism will then be able to grow 

 under the more favourable conditions, and a form which 

 has become completely non-virulent can be made to 

 regain its lost virulence. 



Collodion sacks are now frequently used to study the 

 action upon animals of the dialysable products produced 

 by micro-organisms which do not form any appreciable 

 amount of toxin in vitro, for cultivating species which 



