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Pregnacy - Embryo at 4 (Four) Weeks,  28 day  Embryo in Utero. The uterus, chorionic villi and chorionic sac are cut to allow visualization of the amniotic sac. Embryo has a  C-shaped curve and measures 4.0 mm CR (crown-rump length).   The upper limb buds are like flippers and the lower limb buds have begun to form. Four pairs of branchial or pharyngeal arches are visible and the lens placodes  (primitive eyes) are distinct.  The heart prominence is also easily recognized.