Ostergren (1951) provided a simple explanation for both chromosome distribution in mitosis and chromosome segregation in meiosis, and more recently a molecular extension of his hypothesis has been possible. This report focuses on experimental tests of these ideas. Micromanipulation experiments on cell hybrids containing both meiotic and mitotic spindles demonstrate that differences in meiotic and mitotic chromosome behavior are determined by something intrinsic to the chromosome: meiotic chromosomes transferred to a mitotic spindle (or vice versa) behave just as they normally would. The molecular explanation postulates polarized growth or binding of microtubules at kinetochores. This has just been tested in vitro by McGill & Brinkley (1975) and by Telzer, Moses & Rosenbaum (1975), and their results are reviewed. In addition, a novel method for in vitro studies is described - mechanical demembranation of cells which is compatible with quite normal chromosome movement in anaphase. After addition of microtubule subunits to a demembranated prophase cell, chromosome orientation and movement toward an aster was observed for the first time in vitro. It is concluded that important aspects of chromosome distribution are probably understood at both the cellular and molecular levels, but final tests are still required. The outlook is hopeful indeed because the gaps in our knowledge are well known - the necessity of observations on prophase is a recurrent theme - and the means of filling the gaps are in hand.