AbstractBecause its differentiation can be channeled into different pathways, amphibian gastrula ectoderm is a convenient test system for studying factors that control embryonic differentiation.1Chemical nature of inducing factors: A substance that induces muscle and notochord in ectoderm has been isolated from chick embryos and other sources. The factor is protein in nature (mol. wt. in 6 m urea 25,000‐‐30,000). Neural tissue is induced by a crude ribonucleoprotein fraction. Purified RNA has only a very weak inducing activity. The inducing factors are preferentially located in cytoplasmic particles.2Mechanism of action: Embryonic induction has to be considered as a derepression. Preliminary experiments have shown that a high‐molecular‐weight, water‐soluble substance takes part in the inhibition of mesodermal differentiation. The inhibition of differentiation is released by the inducing factors. A close relationship between differentiation and RNA synthesis has been revealed by experiments with actinomycin D (0.5‐‐2.5 μg/ml), which inhibits RNA synthesis. If RNA synthesis is completely stopped in the gastrula stage, the mesodermal area, which is already determined to differentiate into muscle and notochord, still forms some notochordal cells and myoblasts. The differentiation of neural tissue, however, is completely inhibited. DNA‐RNA hybridization experiments at the saturation level suggest that new messenger RNA species are synthesized if differentiation proceeds. But this does not exclude that the inducing factors exert control primarily at the leve