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The factors of seedless expression in Citrus kinokuni hort. ex Tanaka ‘Mukaku Kishu’, ‘Southern Yellow’ {‘Tanikawa Buntan’ pummelo [an uncertain hybrid with C. maxima (Burm.) Merr.] × ‘Mukaku Kishu’}, and the hybrid seedling BSY lines of ‘Southern Yellow’ × Bu1-7 pummelo [chance seedling of ‘Suisho Buntan’ pummelo (C. maxima)] were investigated histologically. ‘Mukaku Kishu’, ‘Southern Yellow’, BSY18, and BSY21 are completely seedless, whereas others are seedy. The percentage of abnormal embryo sacs and fertilization in seedless cultivars and lines showed no differences from seedy ones, indicating that the seedlessness of ‘Mukaku Kishu’ and its progenies was not involved in abnormal embryo sacs and unfertilized ovules. In seedy cultivars and lines, embryos developed to the cotyledon stage by 14 weeks after pollination. In the seedless cultivars and lines, however, embryos only developed to the early stages of zygote or embryo development. These results indicate that the seedless expression of the seedless progenies of ‘Mukaku Kishu’ coincides with ‘Mukaku Kishu’ and is caused by an arrested seed development at the early stage.