Human yolk sacs were studied by light, transmission and scanning electron microscopy. Twelve human embryos at Carnegie stages ranging from 13 to 20 (28-49 days of gestation) were used for this research. The series of events which occur in the yolk sac wall during its period of maximum functional activity were recorded. The endodermal epithelium consisted of a single layer of columnar cells which, through cellular proliferation, formed endodermal cords which became cavitated, thereby forming endodermal vesicles. At the peak of yolk sac activity, intercellular spaces became very large and isolated individual endodermal cells. The epithelial cells were characterized by numerous microvilli on their free surface, high pinocytotic activity and by the formation of dense cisternae. Abundant intracellular vesicles fused together to empty their contents into the endodermal vesicles. The luminal surfaces of both intracellular and endodermal vesicles presented microvilli. The endodermal cells were characterized by an abundant granular endoplasmic reticulum, a well-developed Golgi apparatus, numerous mitochondria and glycogen particles. Endodermal vesicles were normally seen opening into the vitelline cavity through an endodermal orifice. The surface of the outer mesothelium was covered by numerous lengthy microvilli which were denser here than in the endodermal layer. A mucus-like material, present on the surface of the mesothelium, showed relatively few alterations during the study period. The mesothelial cells were less rich in organelles and far less active than the endodermal cells. The microanatomy of the endoderm supports the contention that its cells serve as absorptive structures as well as sites of protein synthesis during early embryonic development. Therefore, the endodermal vesicles could function as a pump regulating the fluid volume into the vitelline cavity, thereby avoiding the collapse of the organ due to the absorptive activity of the endodermal cells. Furthermore, mesothelial microvilli together with their mucous material harbor a layer of serous exudate and thus create a lubricated cushion designed to protect the thin mesothelium from frictional damage.