BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



183 



on either side a pleuro- peritoneal canal through which the pericardial and pleuro- 

 peritoneal cavities communicate (Fig. 191). 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 ap- 

 parent constriction at the yolk stalk, 

 and, with the development of the um- 

 bihcal cord, the peritoneal cavity is 

 finally separated from the extra-em- 

 bryonic coelom. Dorsally, the pleural 

 and peritoneal cavities are permanently 

 partitioned lengthwise by the dorsal 

 mesentery. 



The septum transversum in 2 mm. 

 embryos occupies a transverse position 

 in the middle cervical region (Fig. 192, 2). 

 According to Mall, it migrates caudally, 

 its ventral position at first mo^dng 

 more rapidly so that its position be- 

 comes oblique. In 5 mm. embryos (Fig. 

 192, 5) it is opposite the fifth cervical 

 segment, at which level it receives the 

 phrenic nerve. In stages later than 7 

 mm. the septum migrates caudad, until 

 at 24 mm. it is opposite the first lum- 

 bar segment. During this second period 

 of migration its dorsal attachment travels 

 faster than its ventral portion. There- 

 fore, it rotates to a position nearly at right angels to its plane in 7 mm. embryos 

 and its original dorsal surface becomes its ventral surface. 



The Pleuro-pericardial and Pleuro-peritoneal Membranes. — The common 

 cardinal veins (ducts of Cuvier) on their way to the heart curve around the pleural 

 cavities laterally in the somatic body wall (Figs. 191 and 193). In embryos of 

 7 mm. each vein, with the overlying mesoderm, forms a ridge which projects 

 from the body wall mesially into the pleural canals. This ridge, the pulmonary 

 ridge of Mall, is the anlage of both the pleuro-pericardial and pleuro-peritoneal 



Fig. 192. — 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. 



