Nucleon electromagnetic form factors and chiral singularities

Chiral singularities in both nucleon isovector radii are calculated in clos ed form. Within a one-meson-loop approximation we implement in an explicitl y gauge invariant manner quark substructure in the gamma pi pi and pi NN ve rtex functions. The coefficients of log m(pi) and m(pi)(-1) terms are direc tly related to g(A) and to the threshold behaviour of the pi N amplitude A( (-))(nu, 0) which are 1 and 0, respectively, if only the N pole is consider ed in the pi N scattering amplitude. This result is remarkably independent of the cut-off procedure. If mesonic substructure at the gamma pi pi vertex is included, however, g(A) becomes slightly smaller than 1 and [partial de rivative/partial derivative nu A((-))(nu, 0)](nu=0) becomes moderately nega tive. This hints at important contributions to both g(A) and [partial deriv ative/partial derivative nu A((-))(nu, 0)](nu=0) from the Delta(1232) and p ossibly other N*, Delta* resonances in this approach.