The communication between the peritoneal cavity and the extraembryonic coelom disappears as the yolk stalk obliterates and the amnion fuses with the umbilical cord.
The peritoneal cavity is continuous with a large chamber in the proximal part of the umbilical cord called the umbilical coelom, which surrounds the herniated portion of the midgut loop.
PLEURAL AND PERICARDIAL CAVITIES (THORACIC PORTION)
As each primitive lung sac expands, it encroaches on the pericardioperitoneal canal converting the canal into a primitive pleural cavity. On each side the cavity is separated from the more ventral pericardial cavity by the pleuropericartfial membrane. This membrane begins as a fold of tissue that is pulled from the lateral body wall to the mesocardium by the common cardinal veins when the heart migrates caudally. The membrane gradually narrows the pleuropericardial opening.
Each primitive pleural cavity communicates with the peritoneal cavity through the pleuroperitoneal opening. A fold called the pleuroperitoneal membrane forms between the mesonephros and the septum transversum and begins to close the pleuroperitoneal opening.
As the primitive lung sacs and body wall expand the primitive pleural cavities increase in size.
Source: Atlas of Human Embryos.