Published online by Cambridge University Press: 15 May 2009
1. Methods are described for altering the susceptibility of the surviving allantois to infection by influenza viruses.
2. Under natural conditions susceptibility is a linear function of age between 10 and 18 days of incubation. The drop is 0·11 log10 unit per day for all strains of influenza tested.
3. In whole eggs this decline is masked by the effect of acid allantoic fluid which prevents infection when the pH falls below 6. The effect is not directed against the virus particle, nor does it do permanent damage to the cell, as surviving tissues are equally susceptible whatever the pH of the allantoic fluid that bathed them in ovo.
4. Surviving membranes can be made less susceptible by incubating the eggs at 35° rather than 38° C.; by maintenance for 24 hr. in vitro; by use of deficient or inappropriate medium. These methods lower susceptibility more for some strains than for others.
5. All treatments which lower susceptibility also increase its variation from egg to egg.