In 4th week of the development the lungs begin to develop into the pericardioperitoneal canals (ie, primitive pleural cavities),* In the beginning of the 5th week two small ridges of mesenchyme called Pleuropericardial folds make their appearance, one fold originates form each lateral wall of the undivided thoracic cavity. These folds grow medially toward each other between the developing heart and lungs. Within the free edge of each pleurop-ericardial folds lies the common cardinal vein and phrenic nerve of its own side. With the growth of lungs, the primitive plural cavities expand ventrally around the heart and, on each side, split the body wall mesenchyme into two parts: (i) an outer layer which becomes the definitive thoracic wall, and (ii) an inner layer which is continuous with the pleuropericardial fold of its own side. The extended pleuropericardial folds are now called pleuropericardial membranes. By the end of the 5th week, the pleuropericardial membranes of the right and left side meet and fuse with each other to form the fibrous pericardium. The pericardial cavity is thus completely sealed off and, henceforth, has no communication with the pleural cavities.