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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