234 



THE MESODERMAL DERIVATIVES 



walled atrium, the thick-walled ventricle, and finally the bulbus 

 arteriosus (bulbus aortae) which leads to the ventral aorta or truncus 

 arteriosus. 



The development of the heart is accomplished within the limited 

 space provided by the surrounding body tissues and organs, so that 

 this organ becomes a reversed "S" coiled tube. With the embryo fac- 

 ing to the right, and looking at it from the right side, the shape of 

 the heart would be something like this: 



Vitelline vein 

 0. Dorsol view 



-Vitelline vein 

 B Dorsol view. 



Vitelline vein 



D. Ventrol view 



Development of the amphibian heart. 



The posterior part of the original tube becomes folded dorsally 

 and anteriorly to the more anterior part of the original tube. Con- 

 versely, the anterior part (which is destined to become the muscular 

 ventricle) is folded ventrally and posteriorly to the other parts of the 



