The livers of human embryos 8-32 weeks of age were studied electron microscopically. The liver cells showed marked differences in ultrastructure in various phases of development. The free polysomes decresed in number and the endoplasmic reticulum appear to undergo structural reorganization from vesicotubular to lamellar type around the third month. 'Simultaneously the smooth endoplasmic reticulum and hemosiderin granules appeared, and the glycogen granules increased markedly. These sturctural changes probably correspond to the start of the embryonic hematopoiesis and the biosynthesis of serum proteins in the liver.