428 THE BABBIT. 



part of the eleventh day, the pericardial cavity becomes shut off 

 from the body cavity by a couple of septa. One of these, which 

 is ventral in position, is formed by a thick transverse fold of the 

 splanchnopleuric mesoblast, immediately behind the heart, and 

 between this and the liver. The second, or dorsal septum is 

 much thinner, and grows forwards from the walls of the 

 Cuvierian veins to the anterior end of the body cavity. These 

 two septa, between them, shut off the ventral and anterior portion 

 of the coelom as a pericardial cavity, distinct from the general 

 body cavity (Figs. 150 and 103, CP). 



2. The Pleural Cavities. 



After the boxing-in of the pericardial cavity, by the dorsal 

 and ventral septa, is completed, the general body cavity still 

 extends forwards as a pair of pocket-like diverticula, dorsal to 

 the pericardial cavity, and along the sides of the oesophagus. 

 Into these pocket-like cavities the lungs hang freely, and the 

 pockets themselves become the pleural cavities. 



As the lungs enlarge, the pleural cavities, which at first lie 

 entirely dorsal to the pericardial cavity, gradually extend down- 

 wards so as to embrace its sides (Fig. 163, CK), and ultimately 

 reaching almost to the mid-ventral wall of the chest. 



3. The Diaphragm. 



The diaphragm is formed from a couple of septa, dorsal and 

 ventral respectively, which arise independently, and are for some 

 time quite distinct from each other. 



Of these, the ventral septum is the thick transverse par- 

 tition already described as forming the ventral part of the hinder 

 wall of the pericardial cavity. 



The dorsal septum of the diaphragm arises, on the thirteenth 

 day, as a transverse fold of mesoblast, which grows downwards 

 from the dorsal wall of the body cavity, just behind the Cu- 

 vierian veins. It has for a time a free ventral edge, crescentic in 

 shape; but it ultimately meets, and fuses with, the ventral septum, 

 or posterior wall of the pericardial cavity, thereby completing 

 the diaphragm, and shutting off the pleural cavities from com- 

 munication with the body cavity. 



