Secondarily retroperitoneal organs once had a mesentery and lost it during development. This happens to certain gut structures during the return of the intestines into the abdominal cavity (discussed later). These organs include the pancreas, duodenum, ascending and descending colons, which are associated with fusion fascia adhering them to the posterior abdominal wall.

In the accompanying movie of a cross-section of an embryo, the kidneys (dark purple) are primarily retroperitoneal in the retroperitoneal space (yellow area). The ascending and descending colon develop, suspended within the peritoneal cavity (white area) by a mesentery, and become secondarily retroperitoneal during development. As the colon fuses to the posterior abdominal wall, the peritoneum is lost, and all that remains is an avascular plane, the "fusion fascia" (purple dotted lines) shown at the end of the movie.