Abstract
The fins structure of the trophoblast from a previllous human ovum is described. The extra-embryonal mesoderm was lined by cylotrophoblasts, which were beginning to form the primary villi. The ultrastructure of these cells was similar to that of the Langhans' cells of the definitive villus. The syncytiotrophoblast was located peripherally and consisted of a mixture of giant cells and syncytial masses. This cell type was well differentiated, with its cytoplasm occupied by large amounts of granular endoplasmic reticulum. Golgi elements, and vesicles. The syncytiotrophoblast had many microvilli and pinocytotic vesicles at the surface, lining the lacunae which contained maternal blood cells. Close contact was observed between the trophoblast and the cells of the maternal glands. The trophoblastic invasion of a maternal vessel is described, and the trophoblastic invasion is discussed.