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Diagram showing earliest observed stage of human embryo
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Human foetus, enclosed in the amnion
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Chorion
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A10615284473
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The surrounds the embryo and other membranes. It consists of two layers: an outer formed by the primitive
Ectoderm or
Trophoblast , and an inner by the somatic
Mesoderm ; with this latter the
Amnion is in contact.
The trophoblast is made up of an internal layer of cubical or prismatic cells, the
Cytotrophoblast or layer of Langhans, and an external layer of richly nucleated protoplasm devoid of cell boundaries, the
Syncytiotrophoblast .
It undergoes rapid proliferation and forms numerous
Process es, the
Chorionic Villi , which invade and destroy the
Uterine Decidua and at the same time absorb from it nutritive materials for the growth of the
Embryo .
The chorionic villi are at first small and non-vascular, and consist of trophoblast only, but they increase in size and ramify, while the mesoderm, carrying branches of the
Umbilical Vessels , grows into them, and in this way they are
Vascularized .
Blood is carried to the villi by the branches of the
Umbilical Arteries , and after circulating through the capillaries of the villi, is returned to the embryo by the
Umbilical Veins . Until about the end of the second month of
Pregnancy the villi cover the entire chorion, and are almost uniform in size, but after this they develop unequally.
The greater part of the chorion is in contact with the decidua capsularis, and over this portion the villi, with their contained vessels, undergo atrophy, so that by the fourth month scarcely a trace of them is left, and hence this part of the chorion becomes smooth, and is named the
Chorion Læve ; as it takes no share in the formation of the placenta, it is also named the non-placental part of the chorion.
On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the
Chorion Frondosum .
