This thesis addresses the problem of making high resolution magnetic measurements. The design and development of a new instrument, optimized for this type of investigation , are described and experiments utilizing this instrument to measure slowly changing fields are reported. The biological system studied was the chick in ovo, from which strong (10-100 pT ), slowly changing magnetic fields were measured during the first few days of incubation . To independent sources were modelled. Using a novel extended source algorithm , a continuous current density distribution in the extra-embryonic membranes was imaged and, by greatly refining the experiment, a localized generator was discovered. There is evidence that this source may be associated with the morphological development of the chick embryo.