The experiments have shown that extracts of tissues and tissue juices, under certain conditions, accelerate the growth in intro of the connective tissue from about three to forty times. This activating power was found in many tissues. It was much more marked, however, with the extracts of embryos, of adult spleen, and of the Rous sarcoma. The power diminished directly with the dilution of the extracts, and appeared not to apply to the tissues of a heterologous animal. The power was reduced when heated at 56° C., and removed when heated at 70° C. It was diminished markedly by filtration through a Berkefield filter and was completely suppressed by filtration through a Chamberland filter.