BURGEFF1 was the first to produce evidence of the formation of a sexual factor by mated strains of the fungi Phycomyces blakesleeanus, Mucor mucedo and possibly all other members of the Mucorales. This substance, which is produced in very small amounts, is held responsible for the induction of the formation of gametangia. It appears that it is not species specific, for M. mucedo exhibits a positive reaction with preparations obtained, for example, from Blakeslea trispora as well as from P. blakesleeanus. Plempel2 supposed that the sexual factor is a mixture of two substances that are produced by the plus and the minus strain of the fungus respectively.