The second variety (X₂) arose from the passage series of X₁ and was carried for approximately 20 passages without definite alteration in its character. It differed from X₁ in that it had the ability to appear as the predominant form occasionally in embryos that had received a limiting infective dilution of virus. Because of this, it seems probable that X₂ reverted to parent types at a slower rate than X₁. Like X₁, this virus was never obtained in pure form and suspensions in which it predominated contained large amounts of M and W, especially the former. Good evidence was obtained that X₂ reverted to M and W virus while multiplying in the chorioallantoic sac.

X₃ was derived from passage of X₂ virus after 32 transfers and can be characterized in several ways: (a) X₃ yielded the only X₃ fluids on passage in eggs. This was in striking contrast to X₂ which could not be passed without giving rise to some fluids in which M and W predominated, (b) The X₃ hemagglutinin was weakly inhibited by all but the highest concentrations of M serum, but was strongly inhibited by W serum. X₂ was readily inhibited by relatively small amounts of both sera. (c) X₃ virus, after several passages at limiting dilutions, was neutralized to a highly significant degree by specific M and W sera. This is not necessarily an essential difference from the behavior of X₂ since technical difficulties (large amounts of M and W in X₂ fluids) may have prevented the demonstration of equally striking double neutralization of this virus, (d) After five limiting dilution passages, the X₃ virus retained the characteristics listed under (a) to (c). It is felt that the foregoing facts justify the conclusion that X₃ is a stable virus which combines some of the specific antigens of two parent types.