DIPHTHERIA. 415 



however, that while the majority of properly made toxins 

 have about the same combining power, they do not 

 necessarily correspond in this particular, because when 

 the cultures are allowed to remain too long in the incubat- 

 ing-oven, a change, that takes place very slowly in the 

 cold, transforms the toxins formed by the bacilli into 

 certain other bodies, which he calls toxoids. This would 

 make no particular difference, the substances all being 

 poisonous, except that the toxoids have different combin- 

 ing powers from the toxins and may, therefore, cause 

 confusion. The toxoids consist of three groups, which 

 he describes as pro-toxoids, because they have a greater 

 affinity for the antitoxin union than the toxins ; syn- 

 toxoids, which have an equal affinity for the antitoxin ; 

 and epi-toxoids, which have less affinity for the antitoxin 

 than the toxin. 



The existence of these bodies can be determined by 

 finding the exact limits of toxin-antitoxin neutralization 

 and toxin-antitoxin fatality. The point at which a 

 mixture of toxin and antitoxin is inactive he describes 

 as L ; that at which such a mixture becomes fatal by the 

 addition of a little more toxin as L+. 



The difference between L and L,+ should exactly equal 

 one minimum fatal dose of toxin, but only does so when 

 no excess of epitoxoid is present. When epitoxoids are 

 present and have to be displaced by the added toxins, the 

 difference between L and L+ becomes enormous. Thus, 

 Ehrlich investigated one fresh active toxin and found L 

 = 50 doses of toxin, L+ = 100 doses of toxin, the differ- 

 ence between L and h+ not being one single minimum 

 fatal dose, but fifty of them. From this it will be seen 

 that all calculations based upon L or upon the exact 

 neutralization of the toxin by the antitoxin must be 

 erroneous, because the combining powers of the antitoxin 

 are by no means exhausted in such a mixture. L 1 

 should, therefore, always be determined and made use 

 of as the test-dose. 



The determination of L+ must depend, however, upon 



