Wikipedia
Body-stalk
The body-stalk, also known as the allantoic stalk, is a band of mesoderm that connects the caudal end of the embryo to the chorion in development. 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 hind-gut. With continued development, the body-stalk is later replaced by the umbilical cord.
Body stalk anomaly occurs in approximately 1 in 15,000 births. It is a result of defects in the formation of cephalic, caudal, and lateral embryonic body folds.