In a previous paper, we showed that any Jacobi field along a harmonic map from
the 2-sphere to the complex projective plane is integrable (i.e., is tangent to
asmooth variation through harmonic maps). In this paper, in contrast, we show
that there are (non-full) harmonic maps from the 2-sphere to the 3-sphere and
4-sphere which have non-integrable Jacobi fields. This is particularly
surprising in the case of the 3-sphere where the space of harmonic maps of any
degree is a smooth manifold, each map having image in a totally geodesic
2-sphere.