15. Further studies with chromosome 10. Longley (1937, 1938) discovered that certain strains of maize as well as teosinte have an abnormal type of chromosome 10. It differs from the normal in that it has a very considerable piece of chromatin attached to the end of the long arm. Since the locus of R is known to be in the distal 22 percent of the long arm (Stadler, 1933) it should be possible to determine the amount of recombination between R and the end of the long arm if the extra piece is used as a marker. Dr. Longley was kind enough to furnish a strain with the abnormal tenth. His strain proved to be homozygous for recessive r and a ratio of 1 R : 1 r resulted when pollen from two different strains of R r constitution was applied. Plants from the colored seeds of each F₁, heterozygous for both R and the abnormal tenth, were backcrossed reciprocally by r testers with normal chromosomes 10. The following results were obtained (since the two F₁'s gave similar results they are considered together): When the F₁ plants were used as the female parent the ratio of R : r was 2676 : 7214 while the reciprocal gave close to the expected 1 : 1 ratio. The shortage of R seeds suggests that the normal chromosome 10 fails to be included in the functioning megaspore. There are at least two possible explanations: (1) competition among the megaspores so that one with the abnormal tenth develops into the embryo sac irrespective of its position in the linear tetrad of megaspores or (2) selective segregation at meiosis so that the basal megaspore receives an abnormal tenth. On either basiss if there are no exceptions, the R class represents crossovers. There was no sterility on the ear proving that the abortion of megaspores cannot be accepted as an explanation. Studies are under way to determine the cause of this unusual ratio as well as to ascertain the recombination value between R and the end of the long arm. In connection with the latter problem it is apparent that the true length of a genetic map can never be had from ordinary linkage studies beoause one never knows how muoh crossing over occurs beyond the most distally placed locus studied. It is only when cytological markers are used, such as terminal knobs, that the total map length can be measured. This has already been accomplished for the short arm of chromosome 9 by Creighton. This investigation is being conducted by Virginia H. Rhoades.