Because of the peculiar properties of the psittacosis-lymphogranuloma group of viruses, the pattern of multiplication in the allantois of the chick embryo of one of their number, meningopneumonitis virus, was studied. This was done by determination of the changes in its infectivity for mice and chick embryos.

Titration of infectivity in embryos proved to be a more sensitive procedure than titration in mice; the latter procedure however, had the advantage of greater simplicity and gave more clear-cut results. The mouse titration method was used in most of the experiments.

Following inoculation of virus into the allantois, there was a slow decrease in infectivity in the allantoic fluids followed by an increase due to appearance of new virus between 24 and 48 hours.

The slope of declining infectivity in the allantoic fluids in ovo was similar if not identical with the slope of decreasing infectivity in allantoic fluids in vitro caused by thermal degradation of virus.

Multiplication of the virus in allantoic membranes was characterized by the following pattern: (a) Increase in infectivity in the first few hours (exact duration of increase depended on concentration of virus in inoculum) due to adsorption of virus. (b) Decrease in infectivity up to about 20 to 24 hours. (c) Increase in infectivity due to appearance of the new generation of virus.

The growth curve of meningopneumonitis is analyzed and the pattern of growth is discussed in the light of the present concepts of viral multiplication.

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