STRUCTURE OF THIRTY-THREE HOUR CHICKS 



The gross shape of the heart and its positional relations 

 to other structures are best seen in entire embryos. The fusion 

 of the paired cardiac primordia establishes the heart as a 

 nearly straight tubular structure. It lies at the level of the 

 rhombencephalon in the mid-line, ventral to the fore-gut 

 (Fig. 19). By 33 hours of incubation the mid-region of the 





pericardial 

 _ _ region of coelom 



_' i. _ epi-myocardium _ _ _ J- _ 



.__._- primordial 



cells of 

 \ V - endocardium 





margin of 

 anterior intestinal 

 portal 



ventral aortic , 



root v x ; 



pericardial 

 / / j ,^' J region of coelom ^ X 



epi-myocardium 

 - endocardium - 



omphalomesenteric 



FIG. 27. Ventral-view diagrams to show the origin and subsequent fusion 

 of the paired primordia of the heart. The lines A, C, D, and E indicate the 

 planes of the sections diagrammed in Fig. 26, A, C, D, E, respectively. 



A, chick of 25 hours; B, chick of 27 hours: C, chick of 28 hours; D, chick of 

 29 hours. 



heart is considerably dilated and bent to the right (Fig. 21). 

 At 38 hours the heart is bent so far to the right that it extends 

 beyond the lateral body margin of the embryo (Fig. 22). This 

 bending process is correlated with the rupture of the dorsal 

 mesocardium at the mid-region of the heart. The breaking 



