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.