Abstract
Up to 12 h after anthesis (HAA), the first cell division in rice zygotes occurred with the formation of a slightly oblique transverse wall, causing the partition of the zygote into an apical cell and a basal cell. At 18 HAA, the partition in the apical cell was brought about by the presence of a longitudinal wall and in the basal cell by a transverse wall, which resulted in the formation of a four-celled embyro. At 24 HAA, the middle cell of the four-celled embryo was partitioned by a longitudinal wall, and the basal cell was again partitioned by a slightly oblique transverse wall. At 30 HAA, the apical and middle regions had four cells each with the further formation of longitudinal walls, so that the embryo became ten-celled. After that, cell division was directed at random. Well-developed cytoplasm existed around nuclei in proembryo cells. The cup-shaped plastids containing starch grains, the cup-shaped mitochondria, and the stratiform type of rough endoplasmic reticula were frequently observed. Many vacuoles were seen in the peripheral cytoplasm. The basal cells were more vacuolated than the cells at the upper side of the proembryo. At 18 HAA, the persistent synergid completely degenerated, involving electron dense materials. The endosperm became cytoplasm rich around the proembryo, especially after the embryo had started to increase in size.
