Anaphase Movements in the Living Cell

Abstract

The anaphase movements of living embryonic cells of the chick in tissue culture have been studied in phase contrast illumination, and in polarized light. Cine photographic records of chromosome movements and spindle changes have thereby been obtained simultaneously. Chromosome separation in the chick is made up of two components a movement of chromosomes towards the spindle poles, and an elongation of the spindle as a whole. The assumption is made that the force acting on a chromosome is directly proportional to the velocity with which the chromosome moves. If this assumption is accepted as an approximation, it is possible to interpret the relationship of chromosome velocity to chromosome separation and spindle-fibre length. This leads to the conclusion that anaphase movement is due, not to repulsion between chromosomes, but to a contractile mechanism operating from the spindle poles. Polarized light studies indicate that the orientation of the spindle material is high at the poles, and weaker at the equator. There is a small aster at either end of the spindle. The centrosomes are interpreted as centres of an orienting force, which build up the contractile mechanism of the spindle from isotropic protoplasm. It is suggested that asters may also be contractile, and cause elongation of the spindle. The inadequacy of the old conception of spindle fibres is discussed in the light of more modern views on molecular behaviour.