With the techniques employed we have not been able to detect any qualitative differences between the particles visible in normal blood and in blood from cases of wilt disease and grasserie. Of the two conditions we can speak more definitely in the case of grasserie, because this we have studied quantitatively as well, that is to say, the particles visible microscopically and ultramicroscopically have been counted without bringing to light any marked difference between normal and diseased blood. This leads us to believe that the virus of wilt disease is probably invisible, and the virus of grasserie almost certainly so, when studied with the optical equipment that we have used, and that further evidence will be necessary before one can accept the chlamydozoa or the Borrellina as the active etiological agents. Although these two polyhedral viruses do not appear to be visibly particulate, it does not follow that other alterable viruses are not organized in this way. Each should be considered on its own merits.

This content is only available as a PDF.