254 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



now opens directly into the cephalic wall of the atrium, the inferior vena 

 cava into its caudal wall (Fig. 261 C). The transverse portion of the 

 sinus venosus, persisting as the coronary sinus in part, opens into the pos- 

 terior 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. 262). It becomes relatively lower during the third and fourth 



Crisla terminalis 

 Sept. 11+ L. valve of 

 sinus venosus 

 Septum I 



Sup. vena cava (opened) 



Inf. vena cava 

 Valve of inf. vena cava. 



Valve of coronary sinus 



Tricuspid valve 



'Foramen ovate 



Aorta 



.Semihmar valves 

 of pulmonary artery 



R. ventricle 



FIG. 263. Lateral dissection of the heart of a 105 mm. human fetus, viewed from the right side. 



X 7- 



months. Its cephalic portion becomes the rudimentary crista terminalis 

 (Fig. 263); 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 opening of the vein, and 

 the smaller caudal portion becomes the valve of the coronary sinus 

 (Thebesian valve). 



The left valve of the sinus venosus unites with the septum II, and, 

 in embryos of 20 to 22 mm. or larger, the two bound an oval opening 

 (Figs. 263 to 265). The bounding wall of the oval aperture is the limbus 

 ovalis. 



