 Another possibility is bladder exstrophy, in which part of the urinary bladder is present outside the body.In normal circumstances, the parietal mesoderm will form the parietal layer of serous membranes lining the outside (walls) of the peritoneal, pleural, and pericardial cavities.The visceral layer will form the visceral layer of the serous membranes covering the lungs, heart, and abdominal organs.