1 84 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



ventral to the gut. This mesodermal partition was termed by His the 

 septum transfer sum. In Fig. 1 9 1 it comprises both a cranial portion (desig- 

 nated "septum transversum"), that is the anlage of a large part of the 

 diaphragm, and a caudal portion, the ventral mesentery, into which the 



liver is growing. 



At first the septum transversum does not extend dorsal to the gut, b 

 leaves on either side a pleura-peritoneal canal through which the pericardial 



and pleuro-peritoneal cavities com- 

 municate (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 

 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. 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 

 moving more rapidly so that its posi- 

 tion becomes oblique. In 5 mm. em- 

 bryos (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 lumbar segment. During this second period of migration 

 its dorsal attachment travels faster than its ventral portion. Therefore, 

 it rotates to a position nearly at right angles 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 



FIG. 192. Diagram showing the 

 change in position of the septum trans- 

 versum (modified after Mall). Numerals 

 indicate the length of the embryo at 

 each position of the septum. The letters 

 and numbers at the right represent the 

 segments of the occipital, cervical, thoracic 

 and lumbar regions. 



