Embryonated eggs are among the most useful and available forms of living animal tissue for the isolation and identification of animal viruses, for titrating viruses, and for quantity cultivation in the production of viral vaccines. The embryo proper, chorioallantoic membrane, yolk sac, allantoic sac, or amniotic sac may be inoculated in hens' eggs of various ages, so that a wide choice of types of tissue is available to fit the characteristics of the virus under study or for special studies. The chorioallantoic membrane is frequently used; in some infections, such as smallpox, vaccinia, and herpes simplex, characteristic lesions are produced which in some cases may resemble those in the natural host. For example, smallpox virus when cultured on the chorioallantoic membrane produces pocks and typical inclusions within the infected cells. When the embryo is inoculated, characteristic skin eruptions appear. Influenza virus, however, when inoculated into the amniotic cavity, does not give rise to pathology like that of the natural infection.