He developed a new solid medium using heated blood serum rather than gelatin as the means of solidifying. This medium could now be incubated at 37°C, or body temperature. The Klebs bacilli grew well under these conditions. When they were injected into animals, Loeffler found that the guinea pig developed tissue lesions very similar to those of human diphtheria. Bacilli could be easily recovered from the infection produced at the site of inoculation, but they were never recovered from the damaged internal organs. Loeffler thus postulated that this, too, was similar to human diphtheria, in which the bacteria were confined to the throat membrane. He reasoned that perhaps the bacteria released a poisonous substance that reached other parts of the body through the bloodstream. This supposition was soon proved correct by the work of Émile Roux and Yersin, who did much to reveal the nature of the diphtheria toxin. In practical terms the toxin theory soon bore fruit in the work of Behring and others who developed an effective antitoxin to counter the effects of the soluble poison produced by the bacillus.