THE LATER DEVELOPMENT OF THE CHICK 337 



the blood from the yolk-sac and opening directly into the pos- 

 terior end of the heart. The main vessels of the yolk-sac and 

 allantois are described in the preceding chapter, and we shall 

 need to add little to those accounts of the extra-embryonic 

 circulation. 



1. The Heart 



The sharp bending of the heart to the right divides it 

 roughly into anterior and posterior limbs (Fig. 133, A), and as 

 it continues to elongate an additional loop appears, directed 

 posteriorly from near the apex of the original loop. The entire 

 structure then swings underneath the pharynx and the loops 

 become less widely spread out (Fig. 133, B). The extent of the 

 heart is increased posteriorly by the addition of a region formed 

 by the fusion of the roots of the paired lateral vitelline or 

 omphalomesenteric veins; the chamber thus formed is the sinus 

 venosus. 



During the third day certain constrictions appear, dividing 

 the tube into its primary chambers, and each of these shows 

 characteristic modifications in form, so that by the end of the 

 third day the following regions are distinctly marked. The 

 sinus venosus, formed by the confluence of the omphalomes- 

 enteric veins, is a wide triangular cavity with thin walls; it 

 receives also the embryonic veins, the ductus venosus and the 

 paired ductus Cuvieri (see below). Its apex is anterior, 

 where it opens into the atrium or auricle by the sinu-auricular 

 aperture; this opens into the postero-dorsal region of the auricle. 

 The auricle is formed from the originally posterior loop of the 

 heart tube, now dorsal in position. The auricle already shows 

 signs of its future division in that it is laterally expanded; the 

 sinus venosus is more directly connected with its right side and 

 its left side extends forward nearly to the limit of the peri- 

 cardial cavity. The wall of the auricle remains thin like that 

 of the sinus venosus; its cavity opens downward into the 

 dorsal region of the ventricle. 



The ventricle is formed essentially from the secondary pos- 



