Structure of the mesothelial cells of the human yolk sac was examined with light and transmitting electron microscopy as well as quantitative morphometry methods. In 6-8 week of the embryonic development epithelial cells demonstrate absorptive activity confirmed by numerous microvilli on the apical cell pole, pinocytic vesicles and extended intercell space. By 11-12 week of embryogenesis the epithelial cells function along with some morphometric parameters tend to decrease. The mesothelial cells of human yolk sac might transfer matters from exocoelomic cavity to embryonic blood vessels. Gradual decrease of exocoelomic cavity size is accompanied by involution in its epithelium structure.