The embryonic development of diploid and tetraploidMedicago sativafrom the formation of the zygote to mature seed is described and illustrated. The embryo reaches the global stage at 7 to 8 days; the heart stage at 11 to 13 days; and the torpedo stage at 14 to 16 days after pollination. Full size is attained at about 25 days but food reserves apparently continue to be stored and maturity may not be reached until about the 35th day.Pollination ofM. sativastigmas with pollen from highly incompatible species failed to produce mature embryos. Crosses involvingM. lupulinaproduced no sign of embryo development. However, whenM. arborea, M. blancheana, M. marina, M. platycarpos, M. rigidula, M. ruthenica, orM. scutellatawas used as a pollen parent the early stages of embryonic growth were initiated.