Cystic dilatation of umbilical cord associated with patent urachus.

An urachus is a remnant of allantois in embryo development constituting a connection between the dome of the bladder and umbilicus. It develops on day 16 after conception and closes during the pregnancy to form the median umbilical ligament. Patent urachus results from a failure in closing its lumen in 10-12 gestational weeks.