1. The early reaction to intravenous tubercular infection in the various organs of the rabbit reveals a pathognomonic response in the lungs within 24 hours; the specific response in the liver, spleen, lymph glands, and bone marrow, follows from the 6th to the 14th days.

2. The development and extent of the pathologic process has been analyzed in terms of the activity of monocytes and clasmatocytes.

3. The criteria for differentiating these mononuclear phagocytic cells into two strains have been analyzed and the technics discussed.

4. The clasmatocyte phagocytizes tubercle bacilli freely and fragments them, as it does all cellular and other debris.

5. The monocyte stimulated to metamorphose into the typical epithelioid and giant cell of the Langhans type retains the tubercle bacilli intact, with power to survive and multiply, over long periods of time.

6. The normal number of monocytes or the degree to which monoblasts may be stimulated to development and maturation, together with the activity of the clasmatocytes in destroying bacilli, in any particular region, would appear to be a function of the rapidity and extent of the local tubercular involvement.

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