The descending colon of the embryo, on account of the width of its mesentery, is at first freely movable and lies well over to the left side of the abdominal cavity. In consequence the left layer of its mesentery lies in contact with the parietal layer of the peritoneum. Subsequently fusion of these two peritoneal layers takes place. The fusion begins near the median line, that is, near the root of the mesentery, and extends outward. The fused layers eventually become converted into loose connective tissue. Thus, this portion of the colon loses its mesentery and becomes fixed to the abdominal wall.
This fusion extends usually in the caudad direction as far as the medial border of the left psoas muscle, and takes place to a variable extent in the iliac fossa. Owing to irregularities in the fusion of the mesocolon descendens to the mural peritoneum, transverse folds or septa