Rods and spheres believed to represent viral particles were observed at the free surface of entodermal cells of the chorioallantoic membrane 6 to 44 hours after infection. Although occasional short rods revealed poorly defined internal bodies, the majority, as well as all the longer rods (filaments), exhibited no visible internal structure. The spheres presumed to lie central to the plane of section contained an inner body 20 to 22 mµ in diameter. Both forms possessed a dense, sharply defined limiting membrane 30 A thick and a diffuse external coat of lesser density. Where superimposition within the section was minimal, the viral particles were separated by a relatively constant distance. Measured to include this spacing, on the assumption that it reflected the presence of a component of the outer coat, the diameters of a majority of the rods were 50 to 60 mµ, whereas the spheres averaged 60 to 70 mµ. The rods appeared to form by a process of extrusion from the cell wall and became detached either singly or in bundles of variable length. The spheres seemed to differentiate at the cell surface and to acquire the inner body, limiting membrane, and outer coat as they migrated through the membrane of the host cell. No characteristic changes were seen in the nuclei or adjacent cytoplasm, and recognizable viral particles were never encountered in these areas of the cell. No support was obtained for the assumption that the spheres developed primarily by segmentation of the rods. It is suggested that the spherical form of the virus is the elemental infectious unit and that the filamentous form is largely or completely non-infective.

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