At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the<unk> .
The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta.
As a rule the<unk>  undergoes complete obliteration during the seventh week, but in about two per cent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum, which is situated about two feet above the ileocolic junction, and may be attached by a fibrous cord to the abdominal wall at the umbilicus.
Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the<unk> .
Vitelline duct
Gartner's duct
Urachus
Wolffian duct
Thyroglossal duct
Genu valgum
Cloaca 
Concha bullosa