The communication between the coelomic cavity and the extraembryonic coelom is reduced to a narrow channel on each side of the yolk stalk.
Caudal to the yolk stalk the coelomic cavity is a common chamber ventral to the primitive gut. Cranially it separates into right and left portions that communicate with the pericardial cavity through the pericardioperitoneal canals. A lung bud bulges into the cranial part of each canal as the caudal part narrows because of the expanding hepatic trabeculae and common cardinal and hepatocardiac veins.
The two canals are separated in the midline ventrally by the septum transversum. The septum transversum is an oblique plate of dense cellular tissue between the heart and the hepatic trabeculae. It represents the primitive diaphragm. The coelomic cavity caudal to the septum transversum will become the peritoneal cavity.
Recess on each side of the mesonephric ridge are called medial and lateral coelomic bays.
The entire coelomic cavity is lined with a single layer of cells called mesothelium.
MESENTERIES
Dorsal to the primitive gut the mesothelial layer on each side approaches in the midline to form the dorsal mesentery.
The two mesothelial layers are separated by a thick layer of mesodermal cells in which blood and lymph vessels to the gut and their derivatives develop. Nerves also reach these structures through the mesentery. The splenic primordium originates from the mesodermal cells in the dorsal mesentery in the region of the primitive stomach. The mesentery is named according to the structure it suspends (e.g., mesoesophagus, mesogastrium and mesoduodenum).
The mesocardium migrates to a position ventral to the esophagus, trachea and lung buds.
The mesonephros and gonadal ridge become suspended from the dorsal body wall on each side of the midline by the broad urogenital mesentery.
Source: Atlas of Human Embryos.