Viruses belonging to the psittacosis-lymphogranuloma group have morphological peculiarities which have made them the subject of controversy for many years. They are so large that the desirability of including the group with the viruses has often been questioned. Bedson and his colleages (1-4) described a life cycle for the virus of psittacosis based on the observation of intracellular forms even larger than the elementary bodies. These structures appeared as a graded series of spheres in the cytoplasm, suggesting a high degree of organization and, possibly, binary fission at some stage of development. Dense cytoplasmic inclusions (plaques, morulae) were often associated with the spherical forms.A number of investigations of the morphology of psittacosis and other members of the group have been reported, and the problem was reviewed recently by Meyer (13). I t was felt that a study by means of electron microscopy of virus-infected tissues should provide new information. Consequently, chorioallantois infected with meningopneumonitis virus, a representative member of the psittacosis-lymphogranuloma group, was selected for examination at the electron microscopic level. MethodsInoeulationl.--An allantoic fluid suspension of the "Cal 10" strain (18) of meningopneumonitis virus was inoculated on the chorioallantoic membrane of 12 day eggs. The virus was from the llth egg passage (allantoic cavity) and a heavy dose (0.5 ml. of undiluted fluid having a titer in mice greater than 10 -7) was used in order to have a large number in initially infected cells. Electron microscopic observations of normal membranes were made and described in an earlier study (8).Harvest and Fixation. Girardi, and Allen (18) showed that new virus appeared sometime between 24 and 48 hours after the inoculation of this strain of virus into the al-* Aided by Grants from the Jane Coffan Childs Memorial

The intracellular development of three pox viruses has been studied with the electron microscope using thin sections of infected tissue. Cells infected with vaccinia, ectromelia, and molluscum contagiosum viruses all form developmental bodies preliminary to the production of mature virus. Developmental bodies, believed to be virus precursors, are round to oval, slightly larger than mature virus particles, less dense to electrons, and have a more varied morphology. It is suggested as a working hypothesis that the process of maturation of a virus particle takes place as follows. In the earliest form the developmental bodies appear as hollow spheres, imbedded in a very dense cytoplasmic mass constituting an inclusion body, or in a less dense matrix near the nucleus in cells without typical inclusion bodies. The spheres become filled with a homogeneous material of low electron density. A small, dense granule appears in each developmental body and grows in size at the expense of the low density material. Following growth of the granule, particles are found with the dimensions of mature virus and having complex internal structure resembling bars or dumbells. Mature virus is ovoid and very dense to electrons. An "empty" interior may be found within its thick walls.

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The vacuolating virus SV40 has been recognized as an entity for a relatively short time. Sweet and Hilleman (17, 18) summarized some of the characteristics of the virus but there has been available no information regarding its morphology or intracellular distribution. More recent observations (9) have shown that cells infected with the vacuolafing virus often display well marked, intranuclear, inclusion bodies. The inclusions usually appear in advance of cytoplasmic vacuoles and, as a consequence, it seemed probable that the origin of the virus was in the nuclei of infected cells. For that reason, the following observations were directed toward a study of the nuclear events following infection and, more specifically, toward a description of the virus particle and its intracellular distribution. Materials and MethodsVirus.--Two strains of vacuolating virus (SV,0) were studied. One, PA-57, was isolated by one of us from a patas monkey (Erythrocebus paras) kidney culture in 1957 and carried since that time in paras cultures. Two lines of this strain (including two passage levels of one line) were examined.Strain "CA 45-54 (Sweet and Hilleman), obtained through the courtesy of Dr. Hilleman, was received as the first subculture in grivet monkey (Cemopithecus aettdops) kidney cultures.It was carried in paras cultures in this laboratory.Cells.--Cultures of paras monkey kidney and human kidney were prepared as described previously (10). All cells were seeded on coversllps in Leighton tubes.Fixation and Staining.---Cells prepared for light microscope studies were stained as routine with hematoxylin and eosin after fixation in Zenker's solution, as described in the preceding paper. For special purposes, however, preparations were stained with hematoxylin and ensin after fixation in 10 per cent buffered formalin, Camoy's solution, or after Camoy's fixation followed by hydrolysis in hydrochloric acid, as for the Feulgen reaction. The Fenlgen reaction was performed on cells after fixation in Carnoy's solution for varying periods of time, from several minutes to several days, without noticeable differences.

A vacuolating virus isolated from uninoculated patas monkey kidney cultures was found to be serologically identical with SV40, a virus previously found in association with rhesus and cynomolgus monkeys. Detection of the patas virus was facilitated when patas cells were exposed to x-ray treatment. Rhesus monkey and human cells were relatively insusceptible to the virus, although it persisted in these cells for a long period of time. Distinct intranuclear inclusions were detected in infected patas cultures 2 to 3 days before cytoplasmic vacuoles were noticeable. Cultures previously infected with PA-57 virus did not affect yields of poliovirus, and doubly infected cells were easily distinguished in stained preparations.