|A 3-day-old newborn has difficulties breathing. A CT scan of his chest and abdomen reveals the absence of the central tendon of the diaphragm. Which of the following structures failed to develop normally?

The septum transversum is a thickened layer of mesoderm that gives origin to the central tendon of the diaphragm. It is situated between the thoracic cavity and the omphaloenteric duct. As the lungs grow into the pericardio-peritoneal canal, they give rise to two folds: the pleuroperitoneal and pleuropericardial folds. The pleuroperitoneal folds are responsible for formation of the posterolateral aspect of the diaphragm, and the pleuropericardial folds develop into the fibrous pericardium. The crura provide origin of the dorsal mesentery of the esophagus, whereas the cervical myotomes are responsible for the musculature of the diaphragm. (Note that these are cervical myotomes C3 to C5, the levels of origin of the phrenic nerve.)