Electron-microscopic study of purified preparations, derived from central nervous system tissue of mice and cotton rats by differential centrifugation, has shown that particles alike as to size and shape are obtained from both normal and poliomyelitic tissue. The total yield of nitrogenous material has been essentially the same from both normal and infected tissues.

Characteristic particles separated from both normal and infected tissues are approximately spherical and in the size range from 6 to 30 mμ in diameter. Aggregates of the particles are often obtained.

No intact rods or long filaments have been detected in any of the preparations, but some of the aggregations could easily be mistaken for such forms in unshadowed micrographs.

From a critical examination of our work, and that of others, we conclude that there is no evidence that a virus of the poliomyelitis group has ever been unequivocally identified on electron micrographs thus far published.

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