254 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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 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 coro- 

 nary sinus (Thebesian valve). 



Foramen ovale 



R. valve of sinus venosus 

 Inf. vena cava 



<^ ,Sup. vena cava 

 Seplion II 



Aorta 



Semilunar valve of 

 pulmonary arlery 



R. ventricle 



Fig. 262.— Lateral dissection of tlie heart of a 65 mm. human fetus viewed from the right side. X 12. 



The left valve of the sinus venosus becomes continuous with the septum se- 

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

 opening (Figs. 263 to 265). The bounding waU of the oval aperture is the 

 limbus ovalis. 



Closure of the Foramen Ovale.— The free edge of septum I is, in embryos of 

 10 to 15 mm., directed dorsad and cephalad (Fig. 261 C). Gradually, in later 

 stages (Figs. 264 and 265), its caudal and dorsal prolongation grows cephalad and 

 ventrad until its free edge is so directed . Coincident with this change the septum 

 II, with its free edge directed at first ventrad and caudad, shifts until its free 

 edge is directed dorsad and cephalad, and overlaps the septum I (Figs. 261 C 



