370 BACTERIA. 



alterations in the composition of the blood or other fluids of 

 the body could be determined by any methods of chemical 

 analysis that could then be applied. Then came the theory 

 advanced by Chauveau and others : that just as micro-organ- 

 isms when growing in artificial media produced excretory 

 products, the presence of which was inconsistent with the 

 continued life of the organism ; so in the body, bacteria 

 during the course of the disease gave rise to some material 

 which might act deleteriously on their own protoplasm, and 

 which, remaining in the body for a considerable length of 

 time, interfered with the growth of any similar organisms that 

 might in future be introduced. Here, again, these special 

 chemical products could not be detected in the blood, and 

 must have been present in such infinitesimally small quan- 

 tities that it is difficult to see how they could exert any very 

 marked influence on the activity of the bacteria ; whilst, as 

 Fliigge points out, our knowledge of the action of the tissues 

 on foreign bodies of various kinds would lead us to the conclu- 

 sion that any such material would be very rapidly eliminated. 

 Then Grawitz suggested that in any battle between the cells 

 and the bacilli that may occur in the body during the course 

 of a disease, if the cells can but manage to obtain the upper 

 hand and to destroy the bacteria, they should become 

 hardier, as it were, through the training of the contest, their 

 vital energy and assimilating power should be increased, 

 and they should thus become able to deal in a more summary 

 manner with any organisms with which they might afterwards 

 be brought into contact. Then came Buchner's theory of 

 the inflammatory cause of immunity, which offered another 

 explanation, or modification of Grawitz's explanation. He 

 argued that bacteria made their way into the body at certain 

 special points, these points or seats of election differing in dif- 

 ferent diseases, and that in consequence of the development of 

 the bacteria, there was a reactionary alteration, inflammatory 

 in its nature, in the tissues, which fitted them for the future to 

 resist the special organism that had previously made the 

 attack ; this minute alteration in the function of the special 

 cells at the seat of invasion enabling them to resist the further 

 action and invasion of the same organism even at a con- 

 siderably later period. Again based on the same principles 

 as Grawitz's theory came the now celebrated Metschnikoff 

 theory. Metschnikoff holds that the protection against the 



