The virus of yellow fever as it exists in Aëdes aegypti mosquitoes, both in their so-called infective stage and in the intermediate condition termed the "incubation period", is capable of passing through Berkefeld N filters when suspended in normal monkey serum, although earlier investigators have shown that the virus from infective mosquitoes will not do so when suspended in physiological salt solution.

The virus of yellow fever as it exists in mosquitoes behaves with regard to filtration through Berkefeld N filters as does the virus in the blood of infective monkeys.

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