THE FILTERABLE MICROBES 377 



completely healed in about 4 weeks after inoculation. Failure 

 of the inoculation is not a proof of immunity. The vaccination 

 should be repeated until it does take. 



The Virus of Chicken Sarcoma. Rous in 1910 discovered a 

 tumor in a chicken which is histologically a typical spindle-cell 

 sarcoma and which he has been able to reproduce in other chickens, 

 not only by transplantation but also by inoculation of an agent 

 which can be separated from the tumor cells 1 by nitration through 

 Berkefeld filters, as well as by inoculation with tumor tissue which 

 has been dried and powdered and preserved in the dry condition 

 for months. The filterable microbe, or filterable agent as Rous 

 conservatively calls it, is rendered inert by heating at 55 C. in 

 15 minutes, also by the admixture of chicken bile or saponin. 

 Two other sarcomata of the fowl have been shown to be due to a 

 filterable agent by the same investigator. 



Our conceptions of the nature of filterable agents is at present 

 beginning to become more definite. They are no longer re- 

 garded as necessarily beyond the possibility of morphological study 

 and there is good reason to hope that the development of improved 

 methods of study and their careful application may be able to 

 establish not only the important physiological properties of these 

 agents but their form and perhaps to some extent their structure 

 as well. The beginning already made is full of promise for the 

 future. 2 



1 Rons and Murphy: Journ. Exp. Med., 1913, Vol. XVII, pp. 219-231. Pre- 

 vious papers are cited there. 



2 A number of other diseases have been shown to be caused by filterable agents. 

 A brief mention of these together with references to the literature will be found in 

 the article by Wolbach: Journ. Med. Rsch., 1912, Vol. XXVII, pp. 1-25. 



