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Under ordinary conditions musculofascial cross-grafts made between pairs of rabbits of the same strain and species elicited classical host-homograft tissue interactions. When the cross-grafting was done 7 to 10 days after exchange transfusions leading to introduction of about 40 per cent of foreign blood, the classical host-homograft reaction failed to develop. In its stead there was an harmonious interaction characterized by abundant vascularization of each graft, with minimal stromal replacement and without a trace of inflammation. This reaction resembled a common type previously described in cross-grafts made between postparabiotic twins but lacked some conspicuous features of the reaction of an animal to grafts of its own tissues.

Typhus Rickettsiae are found in large numbers in sections of tissue cultures of scrotal sac exudate. Extensive multiplication of the organisms occurs, and new cells become infected. Organisms are seen in cells undergoing mitotic division. The organisms usually become less numerous after the 6th day in vitro, but in one instance organisms were extremely numerous on the 16th and 21st days. Rickettsiae in tissue cultures retain their intracellular location, even when infection is very heavy. They multiply exclusively in non-phagocytic cells which are believed to be of mesothelial origin. Pleomorphism is much more pronounced in tissue cultures than in guinea pig tissues and is entirely comparable to that seen in the louse.

Several methods for the experimental study of the rickettsial diseases have been applied to fièvre boutonneuse. An analysis of the results has indicated that fièvre boutonneuse is a variety of spotted fever and that the etiologic agent is a rickettsia which belongs to the genus Dermacentroxenus and to the species rickettsi (Wolbach (6)). The distinctive morphology of the organism and its characteristic intranuclear clustering in ticks and in tissue cultures are the important criteria upon which this conclusion is based. Immunological, histological, and cytological observations of a confirmatory nature are also reported.

Skeletal myofibrils isolated either by tryptic digestion at 0 degrees C. or by a colloid mill and suspended in buffer solution (pH 7.0, micro; 0.154) containing 20 per cent glycerin and 0.0025 M adenosinetriphosphate at -5 degrees C. contracted slowly and progressively when the temperature was raised above 0 degrees C. Formalin fixation halted this contraction. With the aid of these procedures myofibrils in progressive stages of contraction were then studied with the electron microscope. Electron micrographs showed that uncontracted fibrils isolated by the colloid mill were structurally similar to those described by other workers. Treatment of fibrils with trypsin removed the Z bands and disorganized the I bands. This enzymatic modification of structure did not impair the contractile response. The principal structural changes during contraction consisted of a migration of dense material from the A band into the A-I junction or the Z band, a gradual increase in width of the fibril, a gradual decrease in length of sarcomeres, an apparent increase in the mean diameter of filaments, and a disorientation of these latter from their parallel arrangement.

The temperature at which tissue cultures infected with typhus Rickettsiae are incubated has been shown to exert a marked influence on the intracellular multiplication of Rickettsia prowazeki. At 41 degrees C. the organisms were not found in the cultures histologically on and after the 2nd day in vitro, and the cultures were non-virulent on and after the 3rd day in vitro, in spite of good preservation and growth of the cells. At 37.5 degrees C. organisms were absent from the cultures histologically and the cultures were non-virulent on and after the 11th day in vitro, in spite of good preservation and growth of the cells. At 32 degrees C. good but slow growth of cells occurred and organisms were found in increasing numbers histologically up to about the 21st day in vitro. At this time, 50 to 99 per cent (approximately) of the cytoplasmic volume of the cultures was occupied by Rickettsiae. From the 21st day to the 51st day (the limit to which cultures have been carried successfully) this condition of unrestricted multiplication remained practically unchanged. Inoculation of guinea pigs with single cultures after varying lengths of time in vitro, (up to the 51st day) always resulted in reproduction of typhus in a characteristic manner. At 27 degrees the cell growth was negligible, but many cells remained alive for 10 days or more. Organisms were absent from the cultures histologically and the cultures were non-virulent on and after the 18th day in vitro. The only histological preparations showing unrestricted multiplication of the organisms (infection of the majority of the cells present) were of cultures incubated at 32 degrees C. It is believed that the detrimental effect of the higher temperatures (37.5 degrees and 41 degrees C.) on the multiplication of the organism is exerted indirectly, by stimulation of the defence mechanism of the cells.