DEVELOPMENT OF THE HEART. 



139 



is found to open from the right horn of the sinus into the right part of the common 

 auricle. The sinus now forms a transversely disposed sac, lying below and behind 

 the common auricle, with a larger right and a smaller left horn (the latter being 



b.a. 



I. a. 



l.v. 



Fig. 167. HEART OF HUMAN EMBRYO SLIGHTLY MORE ADVANCED THAN THA.T SHOWN IN FIG. 166. (His.) 



A, interior of auricle and ventricle displayed. 



B, endothelial tube. 



a.c, auricular canal ; a.i, area interposita of His ; m, posterior mesocardium ; r.au, l.au, right and 

 left auricles ; l.v, left ventricle ; r.v, right ventricle ; 6. a, bulbus aortse. 



tapered off into the left duct of Cuvier) ; in this condition it has been termed by His 

 saccus rcuniens (fig. 1 69, B, and fig. 171). The umbilical and vitelline veins soon open 

 into it by a common trunk, which becomes the upper end of the vena cava inferior. 



OLG' 



v.c.s, 



Fig. 168. HEART OF RABBIT EMBRYO. (Born.) 



A, from before ; B, from behind. 



8.8, sinus venosus ; l.v, left ventricle ; r.v, right ventricle ; 6, bulbus aortse; ao', first aortic arch ; 

 ao", second aortic arch ; r.au, right auricle ; l.au, left auricle ; umb.v, umbilical vein ; vi.v, vitelline 

 vein ; v.c.s, vena cava superior. 



The slit-like orifice of the sinus in the back of the right auricle is guarded by two 

 valve-like folds of the endocardium, which project into the cavity of the auricle (right 



