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It is shown that the complete structure of a non-centrosymmetric crystal can be determined from the X-ray diffraction data obtained from the members of an isomorphous series in which two different parts of the structure can be varied independently. The structure so determined may be an enantiomorph of the true structure.

It is shown that the application of Schwarz's and Cauchy's Inequalities to the formulas for calculating Fa** from, respectively, the density of scattering matter and the atomic positions in a crystal leads to relations between the magnitudes of some Fa~,'s and the signs or phases of others. These relations are in the form of inequalities, which vary with the symmetry of the crystal under consideration. A table of the simplest inequalities applicable to crystals possessing each of the simple symmetry elements is included. Examples of the inequalities arising from the presence of combinations of symmetry elements are presented.

SUMMARY1. The distribution of fusimotor axons to bag1, bag2 and chain muscle fibres in cat tenuissimus spindles has been studied using a modification of the glycogen-depletion technique of Edstr6m & Kugelberg (1968). Single fusimotor axons were stimulated intermittently at 40-100/sec for long periods (30-90 sec) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution, and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion.2. Dynamic y axons (i.e. those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibres almost exclusively.3. Static y axons (i.e. those that reduce or abolish the dynamic index) depleted both bag and chain fibres. Bag1 and bag2 fibres were depleted about equally. 4. A single static y axon may activate both bag and chain fibres in one spindle (the most common pattern), chain fibres only in another, and bag fibres only in a third spindle.5. Static y axons with conduction velocities less than 25 m/sec also had a non-selective distribution, but no depletion was observed in bag2 fibres.6. The zones of depletion produced by dynamic y axrons were distributed more or less equally in the intra-and extracapsular parts of spindle poles, whereas those produced by static y axons were mainly intracapsular.7. The results are compared with the glycogen-depletion studies of Brown & Butler (1973 and our own study of the distribution of