32 University of California Publications in Zoology [Vol. 11 



certain of his figures (for example fig. 25) are very suggestive 

 of stages in the development of an organism. He regarded epith- 

 elioma contagiosum and avian diphtheria as separate diseases and 

 held that the lesions found on the mucous membranes of the 

 mouth, nasopharynx, and throat in epithelioma contagiosum were 

 extensions to these surfaces of the lesions found on the skin. Bur- 

 net (1906) made a study of the morphologj' of the disease, but 

 came to no definite conclusion from it. In common with earlier 

 writers he placed the period of incubation at about five or six 

 days. He found the blood and internal organs of infected fowls 

 contained the virus, as shown by the fact that he was able to 

 reproduce the disease by inoculation of healthy fowls with the 

 blood and liver of diseased ones. He also produced the disease 

 by allowing fowls to ingest the virus, and by intravenous injec- 

 tion, in each of which methods the lesions appeared at the usual 

 points of election and not elsewhere, except in one case where 

 a typical lesion was found at post mortem in the oesophagus of 

 a fowl which had eaten the virus. 



CLINICAL FEATURES 



The clinical features of the disease have also been described 

 at length by many writers and we will but sketch them here, 

 making reference chiefly to a few facts in our observations which 

 supplement those recorded by others. 



Period of Incubation. — As has been generally noted, the 

 period of incubation is five or six days. However, this varies 

 with the mode of infection and the virulence of the infective 

 material. With virus which had not been attenuated in any way 

 lesions often appeared on the comb and wattles in three days or 

 even sooner after scarification. The same virus when injected 

 intravenously required a longer period of incubation. On the 

 other hand, virus which had been attenuated by age, heat, or 

 chemicals required a greatly lengthened period in which to mani- 

 fest itself in a macroscopic lesion. These variations in the length 

 of the incubation period are shown in Table I. with the history 

 of the virus used in each case. 



