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The umbilical arteries and vein: the arteries are remarkably diminished in calibre at the end of twenty-four hours, and obliterated almost up to the iliacs in three days; the umbilical vein and the ductus venosus are generally completely contracted by the fifth day. The foramen ovale becomes obliterated at extremely variable periods, and may continue open even in the adult.
In the vascular system we have a number of useless cords which represent relics of atrophied vessels that were once active as blood-canals the ductus Botalli between the pulmonary artery and the aorta, the ductus venosus Arantii between the portal vein and the vena cava, and many others. The many rudimentary organs in the urinary and sexual apparatus are particularly interesting.
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