The connecting stalk, or body stalk also known as the allantoic stalk is a yolk sac diverticulum, that by the third week of development connects the embryo to its shell of trophoblasts. With the formation of the caudal fold, the body-stalk assumes a ventral position; a diverticulum of the yolk-sac extends into the tail fold and is termed the hindgut. Progressive expansion of the amnion from the umbilical ring (surrounding the roots of the vitelline duct and connecting stalk) creates a tube of amniotic membrane. The amniotic membrane and its contents form the umbilical cord that connects the embryo and the placenta.
The connecting stalk is derived from extraembryonic mesoderm. The root of the connecting stalk contains the allantois as a diverticulum of hindgut endoderm.
Anomalies are usually referred to as body stalk anomalies and occur in approximately 1 in 15,000 births. They are due to defects in the formation of the cephalic, caudal, and lateral embryonic body folds, that result in a reduced or absent umbilical cord.
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