The explosion of the use of paramagnetic probes to study biological problems by nuclear magnetic resonance (n.m.r.) stems from the large and easily quantifiable perturbations they produce. The main reason for these perturbations is that the magnetic moments of unpaired electrons are about 103 times greater than nuclear magnetic moments and consequently they generate much greater local fields.Such fields can give rise to large shifts in nuclear resonances; but if these fields fluctuate at the appropriate frequencies they will also cause very efficient relaxation in the nuclear resonances. It is possible to select probes that result mainly in one of these parameters being perturbed so that the terms shift and relaxation probe are now well established in the literature.

Small-angle neutron scattering studies of biological macromolecules have developed rapidly in recent years due to the advent of high neutron-flux reactors,† efficient position-sensitive area detectors for neutrons (Allemand et al. 1975) and novel design of instruments (Ibel, 1976). At the same time position-sensitive detectors for X-rays (Gabriel & Dupont, 1972) have revitalized the small-angle X-ray scattering field (Kratky & Pilz, 1972; Luzzati et al. 1976) and it is now becoming clear that neutrons and X-rays can provide complementary structural information on biological complexes.

The form of multicellular animals and their organs is mainly defined by the curvature of cell layers. They are boundaries for solid tissues; and some organs and organisms consist mainly of distinct cell layers (Fig. I a). The form of adult organisms results from a complex interplay of tissue evagination, growth patterns, production of and interaction with extra-cellular material, and other effects; but the rudiments and basic features of the forms produced can often be traced back to processes of evagination or invagination of nearly flat cell sheets at defined locations in the course of embryogenesis.