1. Comparison of the infections of chick embryos by the chorio-allantoic route indicates that Bacterium tularense and Brucella suis, abortus, and melitensis exhibit varying degrees of facultative intracellular parasitism. Pasteurella pestis is adapted to rapid proliferation and spread in the intercellular fluids.

2. In the early stages of infection Bacterium tularense has a marked affinity for growth within ectodermal epithelial cells. Brucella suis and Brucella abortus differ in their selectivity for cells of mesodermal derivation and especially in their effect on vascular endothelium. The strain of Brucella melitensis studied is limited in its intracellular growth to ectodermal epithelium.

3. Many of the features characteristic of these infections in the natural hosts are reproduced in the chick embryo and its membranes.

4. The possible implications regarding the differences in behavior of these microorganisms in relation to the problem of infection and pathogenesis of these diseases are discussed.

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