Sections of Ehrlich ascites tumor cells infected with anopheles A virus revealed particles in the cytoplasm, which were identified as the virus on the basis of their consistent size and internal structure; their high opacity in the electron beam after fixation with osmic acid; the localized occurrence, within a certain time interval following infection, of high concentrations of such particles within the cell; the absence of similar particles in uninoculated tumor cells and cells infected with other viruses, and the appearance of such particles in infected chorioallantoic membrane.

The particles are spheroid, with a transparent 25 to 30 mµ core, surrounded by an opaque complex shell 12 to 17 mµ in thickness. Apparently "hollow," globular components of this shell have outer and inner diameters of approximately 12 mµ and 5 to 7 mµ respectively. The validity of an additional outer envelope is discussed.

Particles are frequently embedded in a matrix, and their alignment and grouping are discussed. Markedly dense populations are found near nuclear concavities. Some infective changes in the cell are described, for example the formation of newly observed virus-induced necrotic regions.

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