This mesodermal septum forms major part of the diaphragm. As described in Chapter 6, the septum transversum appears during the 3rd week of development as an unsplit mass of mesoderm lying cranial to the primitive pericardial cavity in the flat embryo. Formation of the head fold of the embryo during the 4th week of development results in a reversal of the position of the pericardial cavity and septum transversum. The pericardial cavity comes to occupy a ventral position beneath the foregut. The septum transversum now comes to lie just caudal to the pericardial cavity. Here it occupies the space between the foregut, yolk stalk and forms ventral body wall of the embryo. In this location the septum transversum is an incomplete partition between the thoracic and abdominal cavities because the two pleural canals course dorsally through the septum on each side and communicate i with the primitive peritoneal cavity by a pair of large openings. These openings will be closed by the pleuro-peritoneal membranes.
It is to be noted that only the cranial part of the septum transversurn contributes to the formation of diaphragm. The liver grows into the caudal part of the Septum transversum and, I finally, most of the caudal position of 1 the septum is drawn out as a sagittal mesentery which serves as ventral mesentery of the stomach (this process is described in detail in the chapter 6). As already mentioned, the septum transversum form major part of the diaphragm. However, most of the myoblasts that develop in the mesenchyme of the septum transversum emigrate into the pleuroperitoneal membranes.Consequently,major part of the septum transversum forms the non-muscular central tendon of the diaphragm.