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JAMA. 1941;117(13):1099. doi:10.1001/jama.1941.02820390041013.
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In 1928 Duran-Reynals1 of the Rockefeller Institute reported that extracts of normal rabbit testicle added to an injection fluid will greatly facilitate the spread of vaccine virus through dermal tissues and enhance to an extraordinary degree its infectivity and pathogenicity. Later a similar increase in toxicity or pathogenicity was demonstrated on adding testicular extract to diphtheria toxin or to a suspension of Staphylococcus aureus. Accelerated intradermal spread was also demonstrated with carbon particles. The increased dermal permeability apparently does not depend on any active vascular or nervous response, since increased diffusion occurs in excised skin and even in desiccated skin subsequently soaked in water.2

This "spreading factor" is of teleologic importance. It was extracted from semen, leech extract, bee sting and snake venom, with evidence that the seminal extract facilitates the migration and penetration of spermatozoa. Clinical interest in this synergic extract was enhanced by the discovery that


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