The observations of lengths of chromosomes in salivary glands at pachytene and in mitotic and meiotic metaphases are consistent with the observed coiling of the chromosomes in mitosis and meiosis in plants, animals and Protista, and with the doubly-refractive properties of the chromomeres and chromosomes, on various possible assumptions with regard to the packing of the spiral and the internal changes correlated with spiralisation. Further measurements of the kind that are brought together here, as well as advances from the chemical side, should make it possible to discriminate between these assumptions. It seems likely that different states of packing are responsible for the different degrees of mechanical and chemical stability at various stages in the development of spirals. And further that spirals microscopically visible at metaphase may be beyond the range of visibility at an earlier stage in their development.