Abstract
Summary
Electron microscope examination of ultrathin sections of mouse (NIH-Swiss and Af/Dm strain) embryo cells grown in vitro and showing cytopathogenic changes following passage of polyoma virus preparations has been carried out. Spherical particles of 270 Å in diameter and of low electron density have been observed chiefly in the nucleus, also in the cytoplasm and outside the cells. In some particles an internal structure could be discerned, composed of a more dense center of 50 Å in diameter, surrounded by an electron translucent outer zone with outlines of a limiting membrane. Similar particles have also been found in parotid gland neoplasms and mammary tumors induced in NIH (Swiss) mice by polyoma infected tissue culture fluids. Particles of this type have not been observed in uninfected control cultures.‡
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