4. Plants which have 20 chromosomes plus the short arm of chromosome 5 are intermediate in appearance between disomes and trisomes for chromosome 5. The fragment has a terminal insertion region as the break occurred exactly at the spindle fiber region. In 50% of the cases a trivalent group is formed at metaphase I, and in 50% of the cases a bivalent and the fragment as a univalent are formed. When a trivalent is formed the disjunction in anaphase I is such that the fragment passes to the same pole as one of the normal 5 chromosomes. The two normal chromosomes rarely, if ever, pass to the same pole and fragment plants have never thrown the primary trisome. Through a study of genetic ratios in plants carrying the fragment it has been possible to assign certain genes in chromosome 5 to the long and short arms, respectively. The available data suggest that v₂ ys pr and bt are in the long arm of chromosome 5, while bm₁ and a₂ are in the short arm. Whether a gene shows a 5 : 3 or a 1 : 1 ratio in a back cross using the fragment plants as female determines if a given gene is in the long or short arm.