1. A satisfactory method is described for the topographical study of Rickettsia prowazeki in sections of the scrotal sac of typhus-infected guinea pigs.

2. In such sections Rickettsiae are always intracellular.

3. Typhus Rickettsiae multiply luxuriantly in the serosal cells and produce great distention of these cells.

4. Rickettsiae may be found in small numbers in the endothelial cells lining the underlying capillaries in the testes and scrotum, but are not seen in perivascular macrophages, connective tissue cells, fat cells, smooth or striated muscle fibres or epithelial cells.

5. Rickettsiae are rarely phagocytosed in small numbers by polymorphonuclears but are never seen in lymphocytes, plasma cells or eosinophiles.

6. When infected mesothelial cells proliferate and desquamate, they rapidly lose their content of Rickettsiae.

7. The large mononuclear cells seen in smears of the scrotal sac exudate may be separated into two groups: (1) the serosal (mesothelial) cells which become heavily infected with Rickettsiae but which are not phagocytic for graphite ink and (2) the macrophages (phagocytic cells) which do not contain Rickettsiae.

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