The lateral plates of the intraembryonic mesoderm form the somatic (parietal) mesoderm layer (future parietal serous membrane) and splanchnic (visceral) mesoderm layer (future visceral serous membrane). They border the intraembryonic coelom which is gradually divided into peritoneal, pleural and pericardial cavity.
The septum transversum, a thick plate of the mesodermal tissue between the thoracic cavity and the stalk of the yolk sac, incompletely separates the thoracic and peritoneal cavities ventrally, thus separating the thoracic and abdominal cavities. Pericardioperitoneal canals, on each side of the foregut, leave openings between the two cavities.

1. Lung bud
2. Heart
3. Common cardinal vein
4. Aorta
5. Pleuropericardial membrane
6. Pericardial cavity

1. Pleural cavity
2. Lung
3. Visceral pleura
4. Aorta
5. Pericardial cavity
6. Parietal pleura
7. Superior vena cava
8. Fibrous pericardium
9. Heart

1. Esophagus
2. Hindgut
3. Stomach
4. Tracheobronchial diverticulum
5. Duodenum
6. Midgut loop
7. Septum transversum
8. Cloaca
9. Gallbladder
10. Liver
11. Cloacal membrane
12. Pancreas
13. Heart
14. Ventral mesogastrium
15. Dorsal mesogastrium

With the expansion of the lungs, the mesoderm of the body wall is split into the definitive wall of the thorax and the pleuropericardial membrane. When the pleuropericardial membranes fuse with each other and the root of the lungs they divide the thoracic cavity into the pericardial cavity and the pleural cavity. During further development, the pleuroperitoneal membranes extend in ventro-medial direction and fuse with the mesentery of the esophagus and septum transversum.
Muscular ingrowth from the body wall penetrate the membranes to form the muscular part of the diaphragm.
The diaphragm is formed of the septum transversum, two pleuroperitoneal membranes, muscular component from the body walls, and mesentery of the esophagus.