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DEVELOPMENT OF BODY CAVITIES SEGMENTATION At the end of third week, the intra embryonic mesoderm on each side of the mid line differentiates into Paraxial portion Intermediate portion Lateral plate. When the clefts appear in the Lateral plate mesoderm, the plates are divided into two layers, Somatic/ parietal layer Splanchnic / visceral layer. The space bordered by these layers forms the intra embryonic / body cavity. At first the left and right sides of intra embryonic cavity are in open connection with the extra embryonic cavity / chorionic cavity, but when the body of embryo folds cephalocaudally and laterally, this connection is lost (after 10TH week). In this manner, a large intra embryonic cavity extending from the thoracic to the pelvic region is formed Division of intraembryonic body cavity The intra embryonic cavity initially is horse shoe-shaped and gives rise to three well defined body cavities during fourth week 1. Pericardial cavity 2. Two pericardioperitoneal canals (future pleural cavities) 3. Peritoneal cavity During head folding, the heart and pericardial cavity move ventro cuadally, anterior to the fore gut; the pericardioperitoneal canals pass dorsolateral to the foregut and dorsal to the septum transversum ( a thick plate of mesoderm that occupies the space between the thoracic cavity and yolk stalk.) A pair of folds / membranes in lateral wall of each pericardioperitoneal canal develops (pleuropericardial and pleuroperitoneal membranes) separating the canals from pericardial and peritoneal cavities respectively. The canals enlarge due to growth of lung buds and are converted into pleural cavities. Differentiation of Intra embryonic Mesoderm, Lateral Foldings and Intraembryonic Coelome (Body Cavity) during weeks 3 and 4 as seen in transverse sections Craniocaudal Foldings and its effects on endoderm-lined cavity (gut) and cardiogenic area during week 3 and 4 as seen in mid sagital sections With the continued growth of the lungs the pleural cavities are pushed forward in the body-wall toward the ventral median line, thus separating the pericardium from the lateral thoracic walls Development of Diaphragm 1. Septum transversum 2. Pleuroperitoneal membranes 3. Dorsal mesentery of oesophagus 4. Muscular ingrowth from parietal body walls The thoracic aspect of the diaphragm of a newly born child in which the communication between the peritoneum and pleura has not been closed on the left side; the position of the opening is marked on the right side by the spinocostal hiatus