262 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



dorsal wall of the right atrium is guarded by two valves (Fig. 253). Along the 

 dorsal and cephalic wall of the atrium these unite to form the septum spurium. 

 Caudally the valves flatten out on the floor of the atrium, but the left valve later 

 becomes continuous with the atrial septum II. In embryos of 10 to 20 mm. the 

 atria increase rapidly in size and the right horn of the sinus venosus is taken up 

 into the wall of the right atrium. The superior vena cava now opens directly 

 into the cephalic wall of the atrium, the inferior vena cava into its caudal wall. 



Crista lerminalis 



Jept. U 

 I valve of 



Sinus venasus 



5 up. Vena ecu/a- {opened.) 



Tor am en 



ovale 



Aorta 



VaLve of 

 Coronary smu 



Tricuspid valve 



Oemi lunar 

 valves of 

 joultn.artery 



Jx.ventrtcle 



Fig. 256. — Dissection of the heart of a 105 mm. fetus from the right side. X 7. 



The transverse portion of the sinus venosus, as the persisting coronary sinus, 

 opens into the posterior wall of the atrium. 



The right valve of the sinus venosus is very high in 10 to 65 mm. embryos (first 

 to third month) and nearly divides the atrium into two chambers (Fig. 255). 

 It becomes relatively lower during the third and fourth months. Its cephalic 

 portion becomes the rudimentary crista terminalis (Fig. 256); the remainder is 

 divided by a ridge into two parts, of which the larger cephalic division persists as 

 the valve of the inferior vena cava (Eustachian valve) located at the right of the 



