THROUGHOUT plants and animals the cell nucleus has a uniformity of structure corresponding to the uniformity of its work. This structure has to reconcile the mechanical requirements of cell division and reproduction with the physiological requirements of heredity and development. Its primary importance has led to its study by a great variety of physical and chemical techniques; indeed, a greater variety than has been brought to bear on any other type of structure. Differential staining, X-ray diffraction and X-ray destruction, double refraction, micro-dissection, micro-incineration, ultraviolet spectroscopy and differential digestion as wellas bulk analysis and the vast magnifications of geneticexperiment have been used. They have all played their different parts in the solution of the problem.