The body cavities of the adult (pleural cavities, pericardia! cavity and peritoneal cavity) are formed by partitioning of the intraembryonic coelom. Formation of the intraembryonic cqe-lom and the effects of embryonic folding on this primitive cavity will be described here briefly.
The intraembryonic coelom is formed in the flat embryonic disc during the 3rd week of development when the lateral plate of the intraembryonic mesoderm in the right and left halves and in the cephalic part of the disc splits into layers (somatic layer and splanchnic layer). Splitting of the intraembryonic membrane results in the formation of a inverted U (ie, n) shaped cavity within the embryonic disc. The limbs of the n lie in the two lateral (ie, right and left) halves of the disc while the bend of the n (connecting the
right and left limb of the n) lies cranial to the prechondral plate (ie, in the car-diogenic area). As already described in chapter 7, this part of the intraembryonic coelom is called the primitive peri-cardial cavity and, ultimately, the heart grows into it. The initial parts of the right and left limbs of the n which flank the foregut will form the pleural cavities and finally lungs will grow into them.The caudal parts of the two limbs of the n communicate broadly with the ex-traembryonic coe-lom at the margins of the flat embryonic disc. Later, these parts of the right and left limbs of the n will lose connection with the extraem-bryonic coelom and will merge together to form the single peritoneal cavity.
Folding the embryonic disc during the 4th week of development brings -about significant changes in the disposition of the intraembryonic
coelom. The pericardia! cavity moves ventrally to lie below the foregut. The pericardial chamber communicates at a right angled bend with the paired pleu-ral cavities which now lie more dorsally. The primitive pleural cavities at this stage of development are called peri-cardioperitoneal canals. These canals pass above the septum transversum on each side of the foregut and open cau-dally into the single peritoneal cavity. As mentioned earlier, the peritoneal cavity is formed by the merger of the caudal portions of the right and left limbs of the original horse-shoe shaped intra embryonic coelom. This merger occurs as a result of the folding of the embryo continues, the peritoneal cavity gradually loses its communication with the extra-embryonic coelom; the last region of the closure is at the site of the developing umbilical cord.
In the adult the intra embryonic coelom is divided into three well defined compartments: (1) a pericardial cavity containing the heart, (2) the pleural cavities containing the lungs, and (3) the peritoneal cavity containing the viscera caudal to the diaphragm.
The division of the intra embryonic coelom into three independent and separate cavities is achieved by the development of three sets of partitions: (1) the unpaired septum transversum, which serves as an early, partial diaphragm, (2) the paired pleuropericar-dial membranes, which form partitions that separate the pericardial from the pleural cavities. (3) The paired pleuro-peritoneal membranes, which fuse with
the septum transversum and contribute toward the formation of the thoracoab-dominal diaphragm.