Abstract
In Yucca flaccida there are thirty chromosomes, five “megachromosomes” and twenty-five “microchromosomes.” The length of the large chromosomes is approximately three microns and that of the small chromosomes four-tenths microns. The large chromosomes have a chiasma frequency of 3.00 or one chiasma for each micron of chromosome length. Chiasma formation is perfectly random. On this basis, the small chromosomes should be frequently unpaired if chiasmata alone hold the chromosomes together. All the chromosomes were found to be paired, however, and examinations of metaphase and anaphase plates revealed normal meiotic divisions. An hypothesis to account for the pairing of small chromosomes is presented and pairing of the fourth chromosome of Drosophila in relation to chiasma formation is discussed. Both the cytological and genetical evidence indicate that DARLINGTON'S theory of chromosome pairing at meiosis is invalid.
