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SUMMARY. / 
(1) In 2 monkeys daily inoculations with vaccine virus ceased to 
produce a positive lesion 5 days after the first inoculation. 
(2) Three monkeys similarly vaccinated failed to react 6 days after 
the first successful inoculation. 
(3) In 4 monkeys inoculated daily with variola virus positive reactions 
were not obtained 4 days after the first inoculation. 
(4) In 2 monkeys inoculated daily with variola virus, an abortive 
lesion followed the inoculation done 4 days after the first one, and in 2 
animals a similar lesion developed from an inoculation performed 3 days 
after the primary inoculation. 
(5) Six of the 8 monkeys subjected to daily inoculations with variola 
virus developed an exanthem. The eruption was first present on the 
ninth day in 2, on the eighth day in 3, and on the seventh day of the 
experiment in 1. 
(6) The interval between the last successful daily inoculation (count- 
ing abortive lesions as positive) and the appearance of the exanthem was 
5 days in 2 monkeys, 4 days in 3, and 3 days in 1. 
(7) Four monkeys reacted to both vaccine and variola virus, simulta- 
neously inoculated, by the appearance of typical lesions which developed 
apparently without influencing one another. ‘Two of these animals devel- 
oped a general exanthem on the ninth day of the experiment. 
DISCUSSION. 
A comparison of the results of daily inoculation of the skin with vaccine 
virus with those following similar inoculation with variola virus shows 
that there is a distinct difference in the time of onset of the immunity. 
In the experiments where vaccine was used the refractoriness to skin 
inoculation, if judged by the day on which the first unsuccessful inocula- 
tion was performed, appeared, on an average, during the seventh day of 
the experiment—that is to say, the seventh daily inoculation, which was 
performed 6 days after the first successful one, fails to show a specific 
reaction. 
In the series where variola virus was employed this refractoriness to 
reinoculation appeared, on an: average, during the fifth day of the 
experiment. 
Without committing ourselves to the exact date of onset of the 
immunity, we may yet assert from this that it is of earlier development 
in variola inoculata than in vaccinia. The determination of the exact 
day of development of an immunity to subsequent skin inoculation can 
not accurately be determined by this procedure. In studying the 
evolution of a vaccine or a variola lesion on the skin, we see that an 
interval of from 72 to 96 hours intervenes between the inoculation and 
the appearance of a process which can be diagnosed by the naked eye. 
