530 EMBRYOLOGY OF THE CHICK 



It will also be noticed that the caudal or ventricular end grows 

 toward the right. 



The physical restriction placed upon the growing heart by the dor- 

 sal bending of the entire embryo, and the pushing in of the yolk dor- 

 sally, plus the fact that the entire embryo (by torsion) comes to lie 

 upon its left side, accounts for the particular shape and direction of the 

 heart's bending. 



As the U-shaped bend continues to grow, the closed portion of the 

 U is forced caudad, and twisted upon itself to form a loop. This forces 

 the atrial (arterial region) portion slightly to the left (that is, toward 

 the yolk) and the conus arteriosus is thrown across the atrial region 

 by being bent to the right (or away from the yolk), and then caudad. 

 The closed portion of the loop is the ventricular region. By this twist- 

 ing process the original cephalo-caudal relations of the atrial and ven- 

 tricular regions have become reversed, the atrial region now lying 

 cephalad to the ventricle. 



Not only has the position of the two regions become reversed, but 

 there is a constriction forming which divides atrium from ventricle (Fig. 

 283). The constriction itself is called the atrio- ventricular canal. 



It is on the fourth day that the bulbus arteriosus becomes attached 

 to the ventral surface of the atrium. The bulbus pressing inward, the 

 atrium grows as a seeming expansion around each side of the bulbus, 

 and it is these lateral growings which indicate right and left divisions 

 of the atrium, which later separate entirely. 



The ventricle has an indication of a right and left division also at 

 this same time, caused by a longitudinal groove appearing on its sur- 

 face. 



The bulbus later divides to form the root of the aorta and the pul- 

 monary artery. 



Though the heart began its formation at the level of the hind-brain, 

 it has come to lie now at a level with the anterior appendage-buds, and 

 as the ventricular portion which is the more unattached, it is this ven- 

 tricular region which extends the more caudad. 



Histologically, the endocardium of a four-day chick is still a single 

 layer of cells, while the myocardium can be distinguished from the outer 

 epicardium. The myocardium is composed of elongated cells which 

 show some resemblance to the muscle cells which they are to form. 

 They are arranged in bundles extending toward the lumen. These bun- 

 dles will become the trabeculae carnae of the adult heart. 



The cells of the epimyocardium are becoming flattened to form the 

 true epicardium, while loosely placed mesenchymal cells lie in the region 

 between endocardium and myocardium near the atrio-ventricular canal. 

 These mesenchymal cells will take part at a later period in forming the 

 various septa which are to divide the heart into chambers as well as the 

 connective tissue frame-work of the valves. 



