Abstract
The morphology and development of four members of the reticuloendotheliosis virus group were studied by transmission electron microscopy. Virions of duck spleen necrosis virus, duck infectious anemia virus, chicken syncytial virus, and reticuloendotheliosis virus strain T are sperical with a diameter of approximately 110 nm. They are covered with surface projections about 6 nm long and 10 nm in diameter. The center-to-center distance of surface projections is about 14 nm. The budding virions contain crescent-shaped electron-dense cores 73 nm in diameter with electron-lucent centers. After release of the virions the cores apparently become condensed to 67 nm in diameter. Virions were found budding at the plasma membrane and into smooth-walled, intracytoplasmic vesicles of productively infected cells. The distribution of budding reticuloendotheliosis viruses on cells appeared random over the cell surface, and occasionally aberrant multiple forms of budding virions were observed. The virions appear to resemble mammalian leukemia and sarcoma viruses more closely than avian leukosis-sarcoma viruses.