Between 600 and 900 ovules are estimated to begin development in single normal potato ovaries. The number of seeds per fruit ranged from 0 to 183 in 1941, from 7 to 472 in 1942.Sectioned ovules from several potato varieties were examined. Estimates of the proportion of aborted ovules from freshly opened flowers or nearly mature buds were as follows: Minn. 75-5: 10%, Earlaine: 15%, Early Ohio: 20%, Irish Cobbler: 30–40%, Sebago: 60%, U.S.D.A. 46000: 80%, Netted Gem: 100%. Pollen from the first two varieties only in this list has been used successfully in crosses. In Netted Gem few gametophytes began development, most of the abortion occurring earlier. In the other varieties a larger proportion of the abortion became evident after gametophyte development had been initiated. In Sebago many embryo sacs were immature in freshly opened flowers. Inexact distribution of chromosomes at meiosis probably accounts for a part of the observed abortion. Sterility genes may be responsible for a part. Premature bud and flower abscission lowers the expressed fertility of all varieties but is more effective in some: e.g. Netted Gem and Early Ohio, than in others, e.g. Minn. 75-5 and Earlaine. Fertilization in Irish Cobbler occurred mainly two to four days after pollination; 50% of ovules in the upper half of the ovary showed endosperm divisions. Evidence of fertilization was seen in less than 5% of ovules of U.S.D.A. 46000 taken four days after pollination. Nutritive cells of the integument became considerably enlarged in many ovules containing aborted embryo sacs.