Abstract: In the late globular proembryos, three regions could be identified, i. e. the cotyledon primordium, the epiphysis and the hypocotyl-hypophysis. In the cotyledon primordia, the mitotic frequency of the cells was comparitively high, the directions of the mitotic planes were mostly perpendicular to the long axis of the embryo, the size of the nucleolus was comparitively large, and the cytoplasm density was high. In the epiphysis region, however, the mitotic frequency of the cells was low, the size of the nucleolus was small, and as the first pair of leaf primordia appeared the mitotic frequency of the cells in that region began to increase. In the hypocotyls hypophysis region the mitotic frequency of the cells as well as the size of the nucleolus lied in between the corresponding values of those of the above two regions, the cytoplasm density was low and the size of the vacuoles was large. As the proembryo continued to develop the direction of the mitotic plane changed gradually, from mostly perpendicular to the long axis of the embryo to mainly inclined, or even parallel to that axis. As a result, the proembryo developed from a heart-shaped embryo into a torpedo-shaped embryo. After the first pair of leaf primordia appeared from the young embryo, the vacuoles in the cells of the cotyledons grew in size rapidly. About twenty to twenty five days after flowering, the starch grains, the protein bodies and the lipid granules began to accumulate in the cells of the cotyledons and gradually increased both in size as well as in quantity. About fifty days after flowering the diameter of the starch grains reached its maximum value of 6.2–7.0 μm, and decreased in value thereafter till the time of harvesting when most of the starch grains disappeared except those in the palisades. On the other hand, fifty to sixty days after flowering, the diameters of the lipid granules and of the protein bodies reached their maximum values of 5.4–7.0 μm and 6.2–7.0 μm, respectively. The observation revealed that the formation of the protein bodies was related to the vacules.

Key words: Soybean, Embrionic development, Cotyledon