HeLa cells in tissue cultures infected with types 3, 4, or 7 of adenovirus (RI-APC virus) were studied in order to correlate certain histochemical and electron microscopic findings.

Adjacent thin (ca. 0.05 µ) and thick (2–4 µ) sections of osmium-fixed, methacrylate-embedded cells were cut; by mapping the sections the same cells could be identified with both the electron and the light microscope.

Intranuclear crystalline aggregates seen with the electron microscope to be composed of ordered arrays of viral particles were found by means of the Feulgen reaction to contain DNA. DNA is therefore assumed to be a constituent of the viral particle. The virus appeared to develop from an osmiophilic Feulgen-negative matrix. Displacement of nuclear chromatin occurred during this process.

A Feulgen-azure staining method was found to permit clear distinction between viral and nuclear (host) DNA in thick sections.

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