Changes in fine structure and reserve materials of the dormant embryos of Lactuca sativa L. var. Grand Rapids were studied during the first 4 days of germination. Activation of cells begins soon after the embryos are soaked and involves a progressive loss of reserve materials, an increased clarity of organelles and membrane systems, and development of structures either absent or present in a highly modified state in the dormant embryo. Cells of the hypocotyl just behind the root tip are first activated. Activation then progresses through the hypocotyl and acropetally in the cotyledons. These results are discussed in relation to physiology of seed germination. Plastids show several patterns of development, depending on the tissues in which they are localized. There is developmental continuity between the dissolution of protein bodies and formation of vacuoles.