STRUCTURE OF THIRTY-THREE HOUR CHICKS 



71 



The gross shape of the heart and its positional relations 

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

 of the jpaired cardiac primordia establishes the heart as a 

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

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

 (Fig. 19). B)^_3j_hours of incubation the mid-region of the 



j/SO^-^^ 



'^ • 



pericardial 

 , region of coelom 



epi-myocardium 



, V'^--^ primordial . ^ , 



t f ^ „ , endocardial 



1 \ « I cells of . . 



I >\ J J- tube 



^^v endocardium 



"^ margin of 



> anterior intestinal 



' portal 



ventral aortic , 

 root 



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, Ai, C, D, E, respectively. 



A, chick of 25 hours; B, chick of 27 hours; (Schick of 28 hours; D, chick of 

 29 hours. 



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

 At 38 hqurs the heait 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 



