Viruses 399 



been developed, including the injection of test material with a 

 syringe and needle directly through the shell into specific cavities 

 or directlv into the yolk sac of the developing chick embryo. 



Fig. 73. Chorio-allantoic membrane infected with vaccinia virus. 

 (Specimen mounted in plastic by Dr. Wolcott B. Dunham, Veterans Ad- 

 ministration Teaching Group, Kennedy Hospital, Memphis, Tennessee.) 



Many vaccines prepared to combat rickettsial and viral diseases are 

 being manufactured with fertile eggs serving as the culture 

 medium. 



DISEASE 



VIRUS DISEASES OF BACTERIA 

 While Twort was working with staphylococcal contaminants 

 isolated from cowpox vaccine in 1915, he noticed that degenerative 

 changes were taking place in some of the colonies of these bacteria 

 growing on his culture plates. Two years later d'Herelle observed 

 that by adding a bacteria-free filtrate, obtained from the feces of 

 patients afflicted with bacillary dysentery, to a young broth cul- 



