374 TEXT-BOOK OF EMBRYOLOGY. 



unite to form a simple tubular structure (p. 222; also Fig. 194), the latter is 

 suspended in the cavity by a mesentery which consists of a dorsal and a ventral 

 part, a dorsal and a ventral mesocardium. By these the cavity is at first divided 

 into two bilaterally symmetrically parts. The mesocardia soon disappear and 

 leave the heart attached only to the large vascular trunks which suspend it 

 in the single pericardial cavity. The early pericardial cavity is simply the 

 cephalic end of the embryonic ccelom and is therefore directly continuous with 

 the rest of the ccelom. As mentioned on p. 373 it does not, however, at any 

 time communicate laterally with the extraembryonic ccelom. 



The communication between the pericardial cavity and the rest of the em- 

 bryonic ccelom is soon partly cut off by the development of a transverse fold 

 the septum transversum. This septum is formed in close relation with the 

 omphalomesenteric veins. These vessels unite in the sinus venosus at the 

 caudal end of the heart, whence they diverge in the splanchnic mesoderm. 



am 



vom 



FIG. 329. Transverse sections of a rabbit embryo, showing how the omphalomesenteric veins (vom) 

 push outward across the ccelom and fuse with the lateral body wall, forming the ductus 

 pleuro-pericardiacus (rp, rpd) ; am, amnion. Ravn. 



They are thus embedded in the mesodermal layer of the splanchnopleure, and as 

 the latter closes in from either side to form the gut, the vessels form ridge-like 

 projections into the ccelom. As the vessels increase in size, the ridges become 

 so large that the splanchnic mesoderm is pushed outward against the parietal 

 mesoderm and fuses with it (Fig. 329). Thus a partition is formed on each side, 

 which is attached on the one hand to the mesentery and on the other hand to the 

 ventral and lateral body walls, and which contains the omphalomesenteric veins. 

 It is obvious that these partitions, forming the septum transversum, close the 

 ventral part of the communication between the pericardial cavity and the rest of 

 the ccelom. The dorsal part of the communication remains open on each side 

 of the mesentery as the ductus pleuro-pericardiacus (dorsal parietal recess of His) 

 (Figs. 329 and 330). 



As the heart develops it migrates caudally, and by corresponding migration 

 the pericardial cavity draws the ventral edge of the septum transversum farther 

 caudally, so that the cephalic surface of the latter faces ventrally and cranially. 



