140 EARLY EMBRYOLOGY OF THE CHICK 



embryonic blood vessels with which it is directly continuous. 

 The epi-myocardial layer of the heart is an outer investment 

 which surrounds and reinforces the endocardial wall. As 

 development progresses the epi-myocardium becomes greatly 

 thickened and is finally differentiated into two layers, a heavy 

 muscular layer, the myocardium, and a thin non-muscular 

 covering layer, the epicardium. 



In the apposition of the paired primordia of the heart to each 

 other the splanchnic mesodeim from either side of the body 

 comes together dorsal and ventral to the heart. The double- 

 layered supporting membranes thus formed are known as the 

 dorsal mesocardium and the ventral mesocardium, respectively 

 (Fig. 26). The ventral mesocardium disappears shortly after 

 its formation, leaving the heart suspended in the body cavity 

 by the dorsal mesocardium (Fig. 26 E, D). Somewhat later 

 the dorsal mesocardium also disappears except at the caudal end 

 of the heart. Thus the heart comes to lie in the pericardial 

 cavity unattached except at its two ends. The cephalic end of 

 the heart remains fixed with reference to the body of the 

 embryo where the ventral aorta lies embedded ventral to the 

 floor of the pharynx, and the caudal end of the heart is fixed by 

 the persistent portion of the dorsal mesocardium and the 

 omphalomesenteric veins. 



The straight tubular condition of the heart persists but a 

 short time. The unattached ventricular region becomes 

 dilated and is bent out of the mid-line toward the embryo's 

 right while the fixed bulbo-conus arteriosus and the sinus 

 venosus are held in their original median position (Fig. 49, 

 A-E). This bending of the heart to form a U-shaped tube 

 begins to be apparent in embryos of 30 hours and becomes 

 rapidly more conspicuous, until by forty hours the ventricular 

 region of the heart lies well to the right of the embryo's body 

 (Cf. Figs. 21 and 22). 



The bending of the heart to the side involves a considerable 

 factor of " mechanical expediency." The initiation of the 

 bending process depends on the fact that the heart is becoming 

 elongated more rapidly than is the chamber in which it lies 

 fixed by its two ends. The fact that the bending takes place to 

 the side rather than dorsally or ventrally may be attributed to 



