A 24-week-old male fetus is evaluated by prenatal ultrasound and found to have an omphalocele. Histological examination of the defect reveals that the fascial defect is covered by a layer of peritoneum and amnion but lacks muscular tissue. The embryologist explains that this defect resulted from incomplete fusion of the lateral body wall folds during weeks 4–8 of gestation. Which of the following embryological structures failed to migrate or fuse properly to result in this presentation?