32 
THIRTEENTH REPORT. 
it should be stated that there are other viruses which, more nearly ap- 
proach the visible organisms. These facts, however, indicate that there 
are living particles far beyond our limits of vision. Whether they are 
to be classed among the bacteria or protozoa, whether some of them even 
possess cell structure is uncertain and perhaps must remain so. 
Ann Arbor, Mich., June, 1911. 
