An appreciation of the embryological development of the urinary system gives insight into both normal anatomical variations and some of the congenital abnormalities that may arise. The human urinary system forms from intermediate mesoderm in a process that gives rise to three different, overlapping embryonic renal systems, beginning in the fourth week of gestation. The first system or pronephroi (singular = pronephros) is rudimentary and non-functioning, consisting of 5–10 paired nephrotomes that form in the embryonic cervical region. These are transient and disappear by day 24 or 25, during which time the next structures are forming.