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The leucocyte reaction in smallpox is striking. In the early skin 
lesions there is an absence of leucocytes, the blood shows a hypo-leucocy- 
tosis, and marrow and spleen show absence of formation of polynuclear 
leucocytes. There is increased activity in both marrow, lymph nodes, 
and spleen, but the differentiation of young cells into polynuclear ones 
does not take place. In the monkey there is none of this. Both the area 
of inoculation and the exanthem shows an abundance of polynuclear 
leucocytes, and the marrow shows a leucoblastic activity. It would be 
most important to know if this were also true of variola inoculata in man. 
The shortness of the period of incubation in monkeys also speaks in 
favor of variola inoculata, but there is no variola vera with which to 
compare it. The incubation period of vaccinia in man varies as compared 
with that of animals. The first thought which arises in endeavoring to 
form an hypothesis in explanation of the difference between variola vera 
and variola inoculata in man is that the inoculation is made into a rela- 
tively resistant tissue, and before the organisms have time to develop suf- 
ficiently and so to infect the blood that an extensive skin eruption is 
produced by embolism, the organisms are destroyed or rendered inert by 
the immune substance. This would satisfactorily explain the mild course 
but not the short incubation period. The incubation period in the mon- 
keys was found to be very definite. It did not materially vary whether 
the inoculation was made in the trachea, or by blowing dried virus into 
the lungs, or by injecting it into the blood. In the inoculated monkeys 
the lesions in the bone marrow and testicle, which we have learned to 
regard as a characteristic feature of smallpox, are absent. In one monkey 
only, in which an abundant exanthem followed intratracheal inoculation, 
a single characteristic lesion was found in a seminal vesicle. We do not 
know that a virus similar to that which produces the infection in variola 
vera is formed in the monkey. The only way this could be proven would 
be by exposure of nonprotected individuals. 
Variola inoculata in the monkey differs from variola vera in the rela- 
tively smaller numbers of the intranuclear parasites which are present. 
The same forms are found as in man, but are so few that a prolonged 
search may be necessary to find them. They were found in greatest num- 
bers in 2 cases, one an Orang utan and the other a Philippine monkey 
inoculated in the trachea. Of course, we know nothing as to the relative 
abundance of intranuclear forms in variola inoculata in man. 
Certain experiments were made in the Philippines with reference to 
the immunity produced by vaccinia as compared with that produced by 
variola inoculata. These experiments were not sufficiently numerous and 
varied to cover the entire field. However, they show certain interesting 
features. The immunity produced by vaccinia is stronger and more fully 
_ protective than that produced by variola inoculata. Further, vaccinia is 
a more potent virus than that of variola. It was found easier to produce 
immunity to variola inoculata than to vaccinia. The evidence is that 
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