THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



197 



outer wall of a thicker mesothelial layer and destined to become the myo- 

 cardium the two walls separated by a considerable space. The organ 

 hangs, as it were, in the primitive pericardial cavity (ccelom), connected 

 V 



Dors, tnesocardivm 



card cum 

 t hell urn) 



fieri ca. ret. 

 Cavity 



'EqdocardLury 

 (Eydotfyetiury) 



FIG. 165. Diagrams showing the two anlagen of the heart and their union to form a single 

 structure; made from camera lucida tracings of transverse sections of chick embryos. 

 In C the ventral mesocardium has disappeared (see text). 



at its cephalic end with the ventral aortic trunk and at its caudal end with 

 the omphalomesenteric veins. 



In all U.ammals thus far studied the principle of development in the 

 earlier stages is essentially the same as in the chick. The double origin 

 of the heart is even more marked because of the relatively late closure, of 



