It is now assumed that the common cardinal veins constrict the intra-embryonic coelom at the transition between the pericardial cavity and the pleural canals. At the inner side of the coelom, a fold (plica pleuropericardialis) is formed that develops further into the pleuropericardial membrane. This closes off the passage from the pericardial cavity to the pleural canals. In this way, three cavities develop from the horseshoe-shaped cranial portion of the intra-embryonic coelom: the pericardial cavity, medially and ventrally; and the two pleural cavities, laterally and dorsally. The pleural cavities expand only when the lungs develop.