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The experiments in this paper show that testicle extract causes India ink particles and those of Prussian blue to spread much more extensively through the intercellular spaces than similar suspensions made with Ringer's solution. Methylene blue inoculated intravenously localizes more extensively in areas previously injected with testicle extracts than in control areas receiving injections of tissue extracts without enhancing power. Kidney extracts have this property to a less degree, whereas spleen extracts and blood serum are devoid of it. The spreading power of extracts is destroyed by heating at 60°C. for 30 minutes, as is also the power to enhance infections. The precise mode of action of the Reynals factor is not known, but the results of the experiments here presented suggest that it may depend at least in part on the property whereby testicle extract increases the spread of injected material and alters the permeability of tissue cells. It is not inconceivable that changes in permeability facilitate the passage of vaccine virus through the endothelial cells of the blood and lymph vessels, and lead to the generalized vaccinia which is of frequent occurrence in the reported results (20). It has been shown that fluids and suspensions of inert particles are spread by the extract.B. tetanus and B. coli exotoxins and trypsin were not enhanced at all in their action despite the fact that they were spread through a more extensive area in the tissues. Viruses, on the other hand, are markedly influenced and in this respect resemble bacteria, not toxins and enzymes. It appears probable that a definite capacity for multiplication on the part of an injected substance is required if its pathogenic effects are to be enhanced. It may be concluded tentatively that the enhancing power of the testicle extract may depend on that property which not only spreads the injected material through a larger area but renders the tissue cells more easily penetrable by the agents.

The Reynals factor promotes the pathogenic action of the viruses of herpes, vesicular stomatitis of horses, Borna disease, and vaccinia. The heightening of virulence is revealed in various ways. The effects of the viruses may be accentuated; or a weak strain converted into a strong one, as in the case of the F. strain of herpes virus; or the power acquired to infect resistant species or tissues, as, e.g., rabbits and the abdominal skin of guinea pigs, with acute vesicular stomatitis. The Reynals factor should serve as an important agent in the study of filterable viruses.

The injection of tumor extracts and their active protein fractions into rabbits induced the formation of precipitins and neutralizing antibodies. When the major portion of proteins in the tumor extract had been eliminated it induced the formation of neutralizing antibodies, but not of precipitins. The tumor agent, more highly purified by removal of the viscous fraction, did not induce precipitins, and only 2 out of the 15 sera gave any evidence of neutralizing bodies. After the removal of the major portion of protein, the extracts showed insufficient interaction with the sera to fix complement.

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