BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



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the development of three membranes: (1) the septum transversum, which sepa- 

 rates incompletely the pericardial and pleural cavities from the peritoneal cavi- 

 ties; (2) the pleuro-pericardial membrane which completes the division between 

 pericardium and pleural cavity; (3) the pleuro-peritoneal membrane which com- 

 pletes the partition between each pleural cavity containing the lung and the 

 peritoneal cavity which contains the 

 abdominal viscera. 



The Septum Transversum. — In em- 

 bryos of 2 to 3 mm. (Fig. 184) the 

 splanchnic mesoderm of the yolk-sac 

 and that of the heart are continuous 

 where the vitelline veins cross from one 

 layer to the other; also where the um- 

 bilical veins course from the body wall 

 to the heart the somatic and splanchnic 

 layers of mesoderm are continuous. 

 Thus, there is formed caudal to the 

 heart a transverse partition filling the 

 space between the sinus venosus of the 

 heart, the gut and the ventral body wall 

 and separating the pericardial and peri- 

 toneal cavities from each other ventral 

 to the gut. This mesodermal partition 

 was termed by His the septum trans- 

 versum. It is the anlage of a large part 

 of the diaphragm. At first it does not 

 extend dorsal to the gut, but leaves 

 on either side a pleuro-peritoneal canal 

 through which the pericardial and 

 pleuro-peritoneal cavities communicate 



(Fig. 183). In embryos of 4 to 5 mm. the lungs develop in the median walls of 

 these canals and bulge laterally into them. Thus the canals become the pleural 

 cavities and will be so termed hereafter. 



On account of the more rapid growth of the embryo, there is an apparent 

 constriction at the yolk-stalk and, with the development of the umbilical cord, 

 the peritoneal cavity is finally separated from the extra-embryonic ccelom. 



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Fig. 185. — Diagram showing the change 

 in position of the septum transversum in stages 

 from 2 to 24 mm. (modified after Mall). The 

 septum is indicated at different stages by the 

 numerals to the left, the numbers correspond- 

 ing to the length of the embryo at each stage. 

 The letters and numbers at the right represent 

 the segments of the occipital, cervical, thoracic 

 and lumbar regions. 



