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The cytopathic changes induced by poliovirus in cell cultures, first described by Robbins, Enders, and Weller (1), have since been observed by many workers. The description of the morphological changes however, has been limited to the late stages in the swollen and rounded cells, when the changes were so pronounced that they could be detected in the unstained preparations. In a preliminary investigation, it was noted that virus production occurred several hours before the cytopathic effect could be detected in living cultures. The possibility was then considered that a finer cytological study might show changes in the early stages of the cycle of virus multiplication in cultured cells. The present paper reports such a study, in which the sequence of intracellular changes occurring in monolayer cultures of monkey kidney cells after infection with poliovirus, has been related to the growth cycle of the virus. As the experiments were designed to determine the temporal relationship existing between the morphological changes in the infected cells and the appearance of newly formed virus within the cells and in the culture fluid, two conditions had to be fulfilled. Firstly, the cells in the culture should be infected within a brief period; ideally, simultaneous infection of all cells should occur. Secondly, a large proportion of the available cells should be infected with the original inoculum, to insure that the changes observed were actually induced by the seed virus. Materials and MethodsSince the synthesis of poliovirus in monkey kidney cells in tissue culture is rapid (complete within 8 hours), a variable lag in the response of the cells, due to a delay in virus adsorption or other factors, could result in a wide variation in the morphological character° istics of the ceils at a given time.

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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 quantitative distribution of the Conn.-5 strain of Coxsackie virus in different tissues was determined by serial titration at intervals after inoculation of 4 to 5 day old mice. High titers were reached by the 2nd day in blood, heart, liver, muscle, intestine, and its contents, and these were maintained through the 8th day, except for the blood, in which the virus level fell earlier. In paralyzed mice, muscle and brain attained the highest titers and it was in these tissues alone that virus persisted through the 9th day of illness. The pathology of the infection has been briefly described. In particular, the evolution of morbid changes in striated muscle was correlated with the concentrations of virus in muscle. Acute muscle necrosis first occurred when there was a peak viral concentration (4th day), and reached maximal intensity on the 8th day. Scattered acute lesions continued to appear while the virus titer remained above 10–4, from the 9th to 12th day. With the decrease in the myositis, there was a concomitant decrease in the incidence of perceptible disease. Inflammation was found to follow upon the development of necrosis, and subsided slowly. Regeneration began very early, became exuberant, and led finally to restitution of the muscle.

The ultracentrifuge has become a well established tool in virus research, because with this apparatus advantage can be taken of the size of viruses to permit the concentration and purification of these substances. The finding of Schultz and Raffel (1) that poliomyelitis virus from infected spinal cords can be sedimented in the ultracentrifuge has recently been confirmed (2, 3). It has also recently been demonstrated that the analogous virus of mice discovered by Theiler (4) can also be concentrated and purified in this apparatus (5, 6).This paper presents an application of the ultracentrifuge to the preparation from stools of purified materials which can be inoculated intracerebrally into monkeys. The adequacy of this method has been compared with that of the intra-abdominal injection of etherized stool suspension in conjunction with the intranasal instillation of the untreated stool suspension. The former method will be referred to as the ultracentrifugal-intracerebral (or u.-i.c.) method; the latter, as the intra-abdominal-intranasal (or i.a.-i.n.) method. Eleven samples of human, monkey, and chimpanzee stools in which virus was suspected, have been tested by both methods, and two series of titrations with human stools have been carried out.