1. Following self-pollination in alfalfa, 34.4 per cent of the fertile ovules collapsed during the first 144 hours as compared with only 7.1 per cent after cross-pollination. 2. Fertile ovule collapse is associated with excessive growth of the inner integument. The first signs of abnormal development occur in the cells adjacent to the megagametophyte on the funicular side of the seed in the region of the vascular bundle. Breakdown of the endosperm follows, and the seed eventually aborts. 3. This type of seed failure is attributed to inability of the endosperm to keep pace with growth in the surrounding tissues of the seed. 4. The number of cells in the embryo up to 6 days after pollination increases in arithmetical progression. There is little difference in rate of growth following self- and cross-fertilization. 5. The number of nuclei in the endosperm increases exponentially during this period. 6. The rate of growth of the endosperm following hybridization is significantly higher than after self-fertiliz...