152 EARLY EMBRYOLOGY OF THE CHICK 



the extra-embryonic. The delayed closure of the ventral body 

 wall in the yolk-stalk region, results in the embryonic and 

 extra-embryonic ccelom retaining their open communication at 

 this point for a long time after they have been completely 

 separated elsewhere. 



The same folding process which establishes the ventral 

 body wall completes the gut ventrally (Fig. 54, C to F) . Mean- 

 while the right and left ccelomic chambers are expanded mesiad. 

 As a result the newly closed gut comes to lie suspended between 

 the two layers of splanchnic mesoderm which constitute the 

 mesial walls of the right and left ccelomic chambers, respec- 

 tively. The double layers of splanchnic mesoderm which thus 

 become apposed to the gut and support it in the body cavity 

 are known as mesenteries. The mesentery dorsal to the gut, 

 suspending it from the dorsal body wall is the primary dorsal 

 mesentery, and that ventral to the gut, attaching it to the ven- 

 tral body wall is the primary ventral mesentery. 



When the dorsal and ventral mesenteries are first established 

 they constitute a complete membranous partition dividing the 

 body cavity into right and left halves. The primary dorsal 

 mesentery persists in large part but the ventral mesentery early 

 disappears bringing the right and left ccelomic chambers into 

 confluence ventral to the gut and establishing the unpaired 

 condition of the body cavity characteristic of the adult. 



In considering the early development of the heart (Chapter 

 IX) the formation of the dorsal and ventral mesocardia was 

 taken up. In their relation to the other mesenteries of the 

 body, the mesocardia are to be regarded as special regions of the 

 primary ventral mesentery. In the most cephalic part of the 

 body cavity, the gut lies embedded in the dorsal body wall 

 instead of being suspended by the primary dorsal mesentery as 

 it is farther caudally (Cf. Fig. 26, E and Fig. 54, F). The 

 ventral mesentery is, however, developed in the same manner 

 anteriorly as it is posteriorly and when the heart is formed it is 

 suspended in the most anterior part of the primary ventral 

 mesentery. The dorsal and ventral mesocardia are the parts 

 of the primary ventral mesentery lying dorsal to the heart, and 

 ventral to the heart, respectively (Fig. 26, D}. 



When the ventral mesocardium, and a little later the dorsal 

 mesocardium, breaks through, the primary right and left cce- 



