86 
be applied to albuminous substances in solution. We can only accept 
it in the sense that by means of the reaction between the antibodies 
and the antigen the poisonous substance is formed. 
When if is necessary to conceive of bacteriolytic antibodies to 
produce solution in order to cause a reaction we must remember that 
the production requires a definite period of time. This led to the 
question of the cause of the period of incubation, the significance of 
which is important. 
The long incubation period in tetanus, as has been demonstrated, 
may be explained by the fact that tetanus poison has a long distance 
to go along the nerve route before it comes into contact with the 
susceptible cells. 
In the case of malaria the interval between the two paroxysms is 
explained by the fact that the symptoms of the disease are produced 
as a result of the particular phase in the developmental cycle of the 
parasite. 
We have a similar conception of the incubation period of a number 
of diseases, for example, measles, smallpox, etc., in which we look 
upon the period of incubation as due to a breeding of the causative 
factor of the disease. 
All of the above theories which account for the period of incubation 
of disease explain this phenomenon by referring it wholly and solely 
to the parasite and its metabolic products and do not take into account 
the factor of the infected organism. Pfeiffer and Wolff, however, in 
explaining the incubation period in infectious processes, take this 
factor into account: The role which the organism plaj^s in the period 
of incubation in the infectious process becomes clearer from the 
studies upon the power of serum to produce a reaction. 
Von Pirquet believes that, in his work on the hastening power of 
reaction in the case of vaccinia, the power which the organism 
plays in the period of incubation is very evident. 
While the general reaction following the first vaccination at the 
very earliest occurs on the seventh day, it appears earlier upon re vac- 
cination. Also, the course of the reaction is changed in the sense of 
a decidedly shortening and weakening of the symptoms. The earlier 
the reaction appears the weaker does the whole process result. 
How can we explain these’ phenomena? If we use the theories 
generally in vogue, that the cause of the infection must first multiply 
to certain numbers in order to be able to produce symptoms, we must 
believe that the cause of the infection develops more quickly and in 
larger numbers in the animal which has been previously treated than 
in the normal animal. We have already inferred that Arloing has 
suggested this possibility. 
On the other hand, we must take into account that in tuberculosis 
and vaccinia, no matter how much of the causes of these infections 
