THE CIRCULATORY SYSTEM 595 



eral plates extend beneath the pharynx and fuse in the midline to form 

 the ventral mesocardium. 



The splanchnic layer of the pericardium becomes folded dorsally so 

 as to enclose the endothelial cells which have formed a more or less 

 short tubular arrangement. Then the folds meet and fuse dorsally to 

 form another tube on the outside of the endothelial tube. These latter 

 tubes thus enclose the endothelial tubes, and are connected with the 

 dorsal wall of the pericardial cavity. The connection forms the dorsal 

 mesocardium. The outer tube forms the muscle of the heart walls. 



Much difficulty will be avoided in our later studies if, after a review 

 of the chick's circulatory system, the following account of the heart be 

 mastered : 



The paired endothelial tubes fuse together cephalad to form the 

 bulbus aortae. Then there are outgrowths which form the beginnings 

 of the truncus arteriosus or ventral aortae. The endothelial tubes do 

 not fuse entirely in the posterior region, and are not alike on each side. 

 The right one is bent and forms the beginnings of the ventricle and the 

 right vitelline vein. 



The left one is slightly longer and larger in diameter in the more 

 naudal portion. This is to become the auricle, while its continuation in 

 a caudal direction will become the left vitelline vein. Both vitelline 

 veins are connected directly with the yolk-mass and the liver. 



As the embryo develops, the two endothelial tubes fuse quite like 

 the letter S, after which the dorsal mesocardium disappears, so that, as 

 in the chick, the heart tube is attached only at its two ends. 



The heart now comes to have its caudal end toward the left side 

 and resting against the liver. This caudal portion is the region of the 

 sinus venosus and the auricles. 



However, as the heart continues growing much more rapidly than 

 the surrounding portions, and as it is attached only at its two ends, the 

 cephalic region of the heart and the ventricle region swing downward 

 and come to lie ventral in position. This naturally forces the auricle 

 dorsad, and the auricle thus comes to occupy the greater portion of the 

 dorsal side of the adult heart. 



Constrictions separate the heart early into two limbs, but it is only 

 after the tadpole's mouth has opened that the auricle is divided into 

 right and left halves by the inter-auricular septum which grows ventrad 

 from the dorsal wall. The sinus venosus remains connected with the 

 right auricle, while the pulmonary veins later enter the left auricle. The 

 pulmonary veins can hardly be seen during the tadpole stage. 



The ventricle walls thicken, and a few days after the mouth opens, 

 the bulbus aortae divide into an anterior and a posterior portion. The 

 anterior portion is called the truncus arteriosus. This truncus arteriosus 

 is also divided into right and left channels. 



