February, '16] CHAPMAN AND GLASER: WILT OF GIPSY MOTH 153 



ments exceed those in the controls by two. In Table IV the deaths 

 in the experiments exceed those in the controls by 15. It was abso- 

 lutely impossible to come to any conclusions on the results based on 

 the above experiments. Table V represents an experiment in which 25 

 animals were infected with fresh, undiluted and unfiltered gipsy moth 

 wilt virus. Twenty-five controls accompanied this experiment. As 

 can be seen the deaths in the contrpls exceeded those in the experiments 

 by six. Altogether fifteen experiments were performed and the re- 

 sults were all very similar to those outlined above. Can such discour- 

 aging results find an explanation, and how can they be made to har- 

 monize with those obtained in 1913? The only feasible explanation 

 which occurred to us was that many of the caterpillars used in our 

 experiments in ,1914 were chronically infected with wilt before being 

 collected and that this accounted for the high mortality in the con- 

 trols. Why some of the animals infected with the unsterilized virus 

 survived and produced moths can possibly be explained by the sug- 

 gestions offered by Glaser and Chapman (1913) and Glaser (1915). 

 In short it seems very likely that many individuals' are naturally im- 

 mune towards wilt. In the discussion of the Berkefelcl filtration ex- 

 periments of 1913 it was further pointed out, that the wilt virus is 

 difficult to filter on account of the abundance of cellular debris, pig- 

 ment granules, hairs and polyhedral bodies. A film very soon be- 

 comes deposited on the outside of the Berkefeld candle and of course 

 this retards filtration. In order partially to overcome this retarda- 

 tion, the virus was diluted to about 100 c.c. of sterile water and filtered 

 through a fine grade of paper prior to filtration through the candle. 

 As is well known, however, the albuminous material, polyhedral bod- 

 ies, etc., easily pass through filter paper so that it is practically impos- 

 sible to obtain material for the Berkefeld filter which will not deposit a 

 film on the candle. Absorption of material into the interior of the 

 candle necessarily also must retard the filtration. Naturally, the old 

 candles must be frequently replaced by sterile new ones even during the 

 course of one filtration. 



As stated previously, the results obtained in 1914 were gravely at 

 variance with those of 1913. Evidently we were fortunate in collecting 

 healthy material in 1913, but our 1914 results clearly demonstrated 

 that our chance method of obtaining healthy material ended disas- 

 terously. Another method had to be devised. 



Origin of Material for the 1915 Experiments 

 If further experimental progress was to be made, it was thought 

 absolutely necessary to raise a stock of caterpillars free from wilt in- 

 fection or at least a stock in which the wilt mortality was reduced to a 



