Thompson, MaxineM. (U. California, Davis.)Cytogenetics of Rubus. III. Meiotic instability in some higher polyploids.Amer. Jour. Bot. 49(6): 575–582. Illus. 1962.—Meiotic instability occurred in low frequency in polyploidRubusvarieties, ‘Young,’ ‘Boysen,’ and ‘Mayes.’ Subdivision of the chromosome complement into 2 or more groups that function independently within the cell, nondisjunction of chromosomes, and unequal distribution of the chromosomes to the polar regions at anaphase were noted at either or both meiotic divisions in the pollen mother cells. The termcomplement fractionationis suggested for the phenomenon of independently operating groups of chromosomes of variable numbers within meiotic as well as mitotic cells. The presence of PMC's with unusual chromosome numbers indicates a pre‐meiotic instability similar to that observed in meiotic cells. With such a cytological mechanism a polyploid organism can produce gametes with extremely variable chromosome numbers. Some of these gametes will be functional due to a tendency for the formation of balanced genomes. The significance of complement fractionation is considered in relation to unusual breeding results, its value as a tool for plant breeders and its role as an evolutionary mechanism. Polyploids are considered as a storehouse of variability which occasionally, by means of the cytological mechanism described, can be released in the form of distinct, new segregants at different levels of ploidy.