Serous cavities — lined by serous membranes (mesothelium)
• all serous cavities arise from one common embryonic coelom
• the separate individual cavities found in the adult are formed by
- inward growth of partitions and
- outward excavation into body wall by cavity expansion

Coelom (celom) — horseshoe-shaped cavity bounded by splanchnopleure and somatopleure
• the original space between trophoblast and hypoblast is filled by lateral mesenchyme which
subseqnently undergoes cavitation cranially and bilaterally, producing a horseshoe-shaped coelom
betweem splanchnic mesoderm and somatic mesoderm
• as a tubular embryo is formed, the coelom is partitioned into intra- and extra-embyonic cavities when
lateral body folds come together along the ventral midline (except at the umbilicus where the
cavities remain connected)
• as the head process grow outward, the cranial loop of coelom (and the adjacent developing heart)
moves ventral to the pharynx and becomes pericardium
• bilateral coelomic cavites are separated by splanchopleure destined to become mediastinum
in the future thorax
• in the future abdomen, splanchnic mesoderm forms dorsal mesentery (ventral mesentery is
largely absent because of the yolk sac)
• surface mesoderm lining the coelom transforms into mesothelium (flattened fibroblasts)

Partitions — grow inward to divide the common intra-embryonic coelom into four serous cavities
• diaphragm formation — the partition between the peritoneal cavity and the two pleural cavities
- a septum transversum contributes the ventral half of the diaphragm
- bilateral pleuroperitoneal folds contribute to the dorsal half of the diaphragm
 (the above partitions produce only the tendinous center of the adult diaphragm)
• pleuropericardial folds — bilateral septae grow to separate one pericardial cavity from
the two pleural canals (a common cardinal vein runs in each pleuropericardial fold)

Pleural cavity excavations — as lungs grow, pleural cavities expand, dissecting into
somatic mesoderm of the body wall
• ventral growth establishes the adult pattern: pleural cavities bilateral to the pericardial cavity
somatic mesoderm is carved into:
- medially: mediastinal pleura & fibrous pericardium, and
- laterally: costal parietal pleura & thorax wall
• caudal growth enlarges the diaphragm, adding a muscular rim that will be used for breathing