A 5-year-old child is brought to the emergency room with massive, painless bleeding from the rectum. Colonoscopy fails to demonstrate a lesion in the colon or anus. Upper endoscopy fails to demonstrate esophagitis, gastric ulcer or duodenal ulcer. A 99mTc (technetium) scan demonstrates an abnormality in the lower half of the abdomen. Failure of a normal developmental process involving which of the following structures is the most likely cause of this child's bleeding?