From a correlation of these studies on the mature fetus, the newborn, and the adult animal, with previously reported findings on the immature fetus, we draw two conclusions: first, that there is a gradually increasing resistance to infection with this virus during intrauterine development, with but little change thereafter; and second, that at the time of birth there is a sudden loss of infectibility by routes other than the intranasal.

These results illustrate then the benefits which may accrue if one projects into the period of antenatal life studies dealing with the age factor in relation to susceptibility to infection. It is implied that data collected from observations of the postnatal animal alone are of necessity incomplete and may be misleading.