Some theoretical aspects of osmium tetroxide fixation with special reference to the metaphase chromosomes of cell cultures.

Fixation of cell cultures with 1 per cent OsO(4) at constant pH and tonicity but variable cationic valence and dielectric constant causes profound changes in metaphase chromosomes. It is possible to make them disappear, flocculate, or show little change from the living cell in the phase contrast microscope. Conventional fixation for the electron microscope causes almost complete disappearance of metaphase chromosomes in phase contrast. Reasons for this behavior are discussed. It is postulated that intermolecular distances and consequently internal structure in chromosomes are governed by the same forces that govern these distances in colloidal sols.