Abstract:The present paper reports the results of the observation on the megagametophyte(embryo sac) development in Panax japonicus C. A. Mey. The megasporocyte undergos meiosis and gives rise to a linear tetrad of megaspores. The development of embryo sac is of the polygonum type. The chalazal megaspore is functional. At the fore-nucleate embryo sac stage, a large quantity of mitochondria, many long or short endoplasmic reticulum (ER) cisternae, some plastids and lipid bodies are present at the micropylar end of it, while at the chalazal end large concentric ER whorls and some mitochondria are seen. In general, at the free nucleate stage of embryo sac development, the quantity of organelles at the micropylar end is more than those at the other end. Small plastids containing starch grains occur in those cells adjacent to the chalazal end of the embryo sac, while the cells nest to the microplar end contain large lipid bodies and large plastids with starch grains. In mature embryo sac, antopodals disintegrate early, polar nuclei have fused into a secondary nucleus. The egg cell has fewer organelles and a large vacuole occupied the micropylar end of it. In contrast, synergid is rich in mitochondria, ER, dictyosomes and contains a well-developed filiform apparatus. Periodic acid-Schiff(PAS) reaction shows that starch grains are accumuiated within the egg apparatus and the central cell. After fertilization, the primary endosperm nucleus (fertilized secondary nucleus) begins to divide into endosperm free-nuclei, and then to form endosperm cells when some dozens to hundreds of free nuclei have reached. The zygote undergoes a long period of dormancy.