EARLY in the development of the ovum the blastocyst cavity is formed surrounded by a single layer of trophoblast cells with the inner cell mass to one side. These cytotrophoblast cells remain both in the placental chorionic villi and in the reflected chorion, though only in early pregnancy is there evidence of syncytium over the chorionic membrane. By the 7th or 8th day after fertilization, the formation of the amniotic vesicle separates the dorsal surface of the germ disc, from which skin is derived, from the celIs destined to form amnion. This close developmental origin of amnion and skin suggests that the two may have many features in common, and that the study of the ultrastructure of fetal skin, amnion and chorion may not only reveal these fundamental resemblances but also indicate functions they all possess.
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