A DISTRIBUTED QUASI-STATIC IONIC CURRENT SOURCE IN THE 3-4 DAY-OLD CHICKEN-EMBRYO

We report measurements of slowly varying magnetic field patterns close to fertilized eggs of the chicken Gallus domesticus during the first few days of incubation. These fields are generated by ionic currents w ithin the egg that are associated with the development of the embryo. Since they are very weak (no greater than tens of pT) and vary over di stances of a few millimetres, it has been necessary to develop special ized instrumentation and analysis techniques. We describe the use of h igh-spatial-resolution SQUID magnetometers to measure the field patter ns and appropriate imaging algorithms to model the current sources res ponsible for producing the fields. Our results provide strong evidence for a distributed source in the extraembryonic membranes. There is al so indication of a more localized source within the embryo itself.