EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD-VESSELS 



263 



opening of the vein, and the smaller caudal portion becomes the valve of the coro- 

 nary sinus (Thebesian valve). 



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. 257 and 258). The bounding wall 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. 254, C). Gradually in later 

 stages (Figs. 257 and 258) its caudal and dorsal prolongation grows cephalad and 



u up. vena cai/a. 

 Aorta 



rulmoncuy 



trunk 



Lcttnu 



Foramen ovale 



Septum I 



nf.vena cava 



Fig. 257. — Dissection of the heart of a 65 mm. embryo, from the left side, showing the septa and the 



foramen ovale. X 8. 



ventrad until its free edge is so directed. Pari passu 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. 254, C, 257, 258). 

 The opening between these septa persists until after birth as the foramen ovale. 

 During fetal life the left atrium receives little blood from the lungs, so that 

 the pressure is much greater in the right atrium. As a result, the septum I is 

 pushed to the left and the blood flows from the right into the left atrium through 

 the foramen ovale. After birth the left atrium receives from the expanding lungs 



