AbstractZebrafish embryos at six developmental stages were exposed to the supernatant of homogenates of zebrafish embryos, 5‐day chick embryos and 9 mm frog larvae. Extracts of embryos bring about developmental arrest of the zebrafish embryo at stages 17–18. The effect is reversible. Arrested embryos returned to buffered water recover and develop normally to hatching, delayed by a time equal to the time of exposure to extracts. Two fractions of the embryo extract are produced by G‐75 Sephadex gel filtration. The inhibitory effect is limited to one of these fractions. The inhibitory factor in embryo extract has characteristics compatible with those of nucleases but the effect on embryos is unlike that of ribonuc