The ultrastructure of the embryo sac of spinach (Spinacia oleracea L. cv. Prevital) has been studied from the inception of the constituent cells to the time of fertilization. The development shows two phases. One is from cell formation until cell maturity and the second from maturity to the fertilizable stage. During the first phase the cell dimensions, areas and volumes of the various cells and cell parts have been measured and compared. The protoplasm of the antipodals hardly increases, whereas that of the egg and central cells multiplies 10 times. But the enlargement of all cells is mainly due to vacuolation. During the second phase the cells develop their final ultrastructure. Initially they are similar – have irregular nuclei, abundant ER, common mitochondria and dictyosomes, scanty plastids and no lipid. The antipodals attain their final structure sooner, are ephemeral and start degenerating. The egg cell differentiates fast, subsequently grows slowly and at maturity a renewed increase, in particular of mitochondria, occurs. The development of synergids and central cel! is gradual. Before fertilization one of the synergids degenerates and in the central cell the polar nuclei form numerous long protrusions which fuse partly. The ultrastructural changes of each cell type have been related to their possible functions. Accumulation and degradation of reserve materials in various cells is discussed in relation with the nutritional supply to the differentiating female gametophyte.