Chick embryos after 7 days of incubation were found to be much more susceptible to infection with vesicular stomatitis virus than were 10 day embryos. They had a 100 per cent mortality and were very suitable for titrations of the virus. The rate of increase of virus in 7 and 10 day embryos was studied. Two different temperatures of incubation were employed, 35–36°C. and 39–40°C., and the growth curves for the virus under the different conditions are presented. 10 day embryos were highly resistant and at 39–40°C. more than half of them survived. At the lower temperature of incubation, 35–36°C., all 10 day embryos died, but they survived much longer than did 7 day embryos.

In the 7 day embryos death occurred after about 12 hours at 39–40°C. and after about 16 hours at 35–36°C., or earlier at the higher temperature.

In embryos of both ages the virus titer reached at the higher temperature was only about 1 per cent of that reached at 35–36°C., even in those that died.

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