1. By the bites of stegomyia mosquitoes carrying neurotropic yellow fever virus, encephalitis has been produced both in white mice and in rhesus monkeys.

2. The fixed neurotropic strain of virus cannot be maintained in the mosquito host as well as can the viscerotropic strains. This is doubtless attributable in part to a smaller amount of virus ingested, because of paucity in the blood stream of the mammalian host.

3. These experiments furnish additional evidence that the long established neurotropic yellow fever virus has changed fundamentally from the parent French strain.

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