28 BULLETIN 431, U. S. DEPARTMENT OF AGRICULTURE. 



nies. When all this is properly done, sacbrood will appear in the 

 inoculated colonies. This shows that the virus 1 of this disease, to a 



Fig. 26.— Berkefeld filter (B) with the glass mantle (A), glass tubing (D), a connecting rubber 

 tubing (C), and a flask (F) with a cotton plug (E). (Original.) 



certain extent, at least, passes through the Berkefeld filter. With 

 this filter the virus is therefore filterable. 



i In referring to the infecting agent in sacbrood, the term "virus" is preferable to the terms "germ" or 

 "parasite." In relation to the disease, however, its meaning is the same as that conveyed by the latter 

 terms. 



