During development of the veins, the first indication of a parietal system consists in the appearance of two short transverse veins, the ducts of Cuvier (or common cardinal veins[1]), which open, one on either side, into the sinus venosus. Each of these ducts receives an ascending and descending vein. The ascending veins return the blood from the parietes of the trunk and from the Wolffian bodies, and are called cardinal veins.
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Image:Gray54.png|Figure obtained by combining several successive sections of a human embryo of about the fourth week.
Image:Gray55.png|Upper part of celom of human embryo of 6.8 mm., seen from behind.
Image:Gray464.png|Dorsal surface of heart of human embryo of thirty-five days.
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