“…This current is carried predominantly by Na+, the transport of which sustains the potential difference across the embryonic epithelium (McCaig & Robinson, 1982). In both developing Xenopus and axolotl, localized, steady, endogenous current leaves the embryo at a location predictive of the future site of limb bud development, but the ions involved have not been identified (Robinson, 1983;Borgens, Rouleau & DeLanney, 1983). Currents of around 20 and 100 1aA/cm2, respectively, leave the primitive streak region in developing chick and mouse embryos (Jaffe & Stern, 1979;Winkel & Nuccitelli, 1989 (Robinson, 1985).…”