CHAPTER XLIII. 



THE CIRCULATORY SYSTEM 

 THE HEART 



Immediately cephalad to the liver, just ventral to the hinder part of 

 the pharynx "(Fig. 328), is the heart region. Here the somatic and 

 splanchnic layers of mesoderm have separated by a wide cavity which 

 is to become the pericardial cavity (Fig. 344), continuous for a time with 

 the general body cavity, but later closed off. 



cnd.cartt. mes.mrti.v. 



Fig. 344. 



Cross sections of frog tadpole. A and B in the region of the anterior and 

 posterior portions of the heart respectively ; C, through a more or less mid- 

 section of a more advanced tadpole, for comparative purposes, ect, ectoderm ; 

 end.card., endocardium ; ent, entoderm ; mes.card.d. and mes.card.v, dorsal 

 and ventral mesocardium ; par, somatic layer of mesoderm (somatopleure) ; 

 per. card., pericardial cavity; vise, visceral layer of mesoderm (splanch- 

 nopleure ) . ( From Rudneff . ) 



Both pericardial wall and muscular wall of the heart are derived 

 from the lateral plate mesoderm, while the endothelium the inner lining 

 of the heart arises from scattered mesodermal cells lying between the 

 splanchnic mesoderm and the digestive tract. These cells can be seen in 

 the two somite stage. 



From Figure 344 one can get a good understanding of how the lat- 



