The histogenesis of the chick yolk sac was studied from incubation day 3 to posthatching day 8. During incubation the epithelial lining forms complex folds which penetrate into the yolk mass. Hemopoiesis is at a maximum between incubation days 11 and 13. The mesoderm is a thin layer during early incubation but thickens in the later stages to form a covering serosa, an intermediate connective tissue layer, and an inner vascularized layer which forms lymphatic tissue after hatching. Involution includes the processes of autolysis and phagocytosis. Protein assimilation, by pinocytosis and phagocytosis, reaches a peak at incubation day 13. Glycogen is present in peak amounts between incubation days 7 and 9. Endodermal alkaline phosphatase activity is localized in the apical cytoplasm between days 7 and 13 and in the basal cytoplasm during the latter incubation stages. Acid phosphatase activity increases throughout incubation to peak levels at day 2 posthatching. Maximum lipid absorption occurs during the latter incubation stages. Ribonucleic acid reveals only moderate staining throughout incubation. Electron microscopy shows that the endoderm is active in pinocytosis and phagocytosis. Lysosomes, rich in acid phosphatase, are numerous within the apical endoderm.