INCUBATION AND DOSAGE. 109 



(precipitating and perhaps other antibodies) are 

 found in considerable concentration in the blood 

 only after the lapse of some days. If the injection 

 of serum given in the first place was of some size, 

 some of the original serum proteids would still be 

 present in the body when the antibody formation 

 had reached a high point, and the conditions for 

 the toxicogenic reaction between the two would be 

 present. This would account for the delayed reac- 

 tion seen in first injections. If some smaller quan- 

 tity were given in the first place the delayed reac- 

 tion might not occur, because the serum proteids 

 would all have been modified or excreted. If a 

 second injection is given, however, the fresh serum 

 at once comes in contact with the antibodies which 

 have already been formed, and the toxic combina- 

 tion or substance can be produced at once. 



On the basis of these considerations von Pirquet 

 believes that the ordinary interpretations of the 

 nature of the incubation period are incorrect. 

 As he says : "I presented the theory that the dis- 

 ease-producing agent only calls forth pathological 

 symptoms in the body when it is changed by means 

 of the antibodies; the incubation period is the 

 period required for the formation of these anti- 

 bodies." This possibility cannot be overlooked 

 in a present-day consideration of this subject, 

 although it is still on a theoretical foundation. 



The number of micro-organisms introduced has 

 an influence on the incubation period. Guinea- 

 pigs may be killed within a few hours by the injec- 

 tion of a large quantity of typhoid bacilli, but with Relation 

 the administration of smaller quantities a much of Dosa * e - 

 longer incubation period may be obtained. The 

 effect of quantity also is shown very clearly by the 



