IQ2 THE ENTODERMAL CANAL AND ITS DERIVATIVES 



Dorsally the pleural and peritoneal cavities are permanently partitioned length- 

 wise by the dorsal mesentery. 



The septum transversum in 2 mm. embryos occupies a transverse position in 

 the middle cervical region (Fig. 185, 2). According to Mall, it migrates caudally, 

 its ventral portion at first moving more rapidly so that its position becomes 

 oblique. In 5 mm. embryos (Fig. 185, 5) it is opposite the fifth cervical segment, 

 at which level it receives the phrenic nerve. In stages later than 7 mm. the sep- 



Com. cardinal vein 



Pleuno -pericardia,L 

 membrane 



Mesonephros 



FIG. 1 86. Reconstruction of a 7 mm. embryo showing from the left side the pleuro-pericardial mem- 

 brane, the pleuro-peritoneal membrane and the septum transversum (after Mall). 



turn 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. In connection with the septum transversum two other membranes 

 develop. 



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



