Herpes simplex virus was visualized by the electron microscope in sections of infected chorioallantoic membrane of chicken embryos. Removal of the embedding methacrylate from relatively thick sections permitted large numbers of viral particles to be seen but caused extensive alteration of cellular components as well as variable distortion of viral structure. This distortion was characterized by disruption of particles and loss of central bodies, resulting in ring or empty shell forms. An inner structure of the virus was revealed in ultrathin sections from which the embedding plastic was not removed. The nuclei of infected cells contained small, dense, primary bodies (30 to 40 mµ in diameter) as well as slightly larger and less dense particles (40 to 50 mµ in diameter) surrounded by a single membrane (70 to 100 mµ in diameter). In the cytoplasm most of the particles possessed a double outer membrane (120 to 130 mµ in diameter). It is suggested that the initial site of viral development is restricted to the nucleus where primary bodies form and become enclosed by a single outer membrane. Upon release into the cytoplasm these particles appear to acquire a second outer membrane and presumably represent the mature virus.

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