98 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



vitelline veins. Of these, the left is small in the liver and later disappears. 

 The right vitelline vein, now the common hepatic, carries most of the blood to 

 the heart from the umbilical veins, and from the Hver sinusoids, gut, and 

 yolk sac. 



Transverse sections of the embryo through the four chambers of the heart 

 show the atria in communication with the ventricles through the atrie-ventricular 



R. common 

 cardinal vein 



L. common 



cardinal vein 



L. vitelline vein 



Left ventricle- 



Bulhus cordis 

 R. atrium 



/ 



R. vitelline vein 

 R. ventricle 



Fig. 100. — Dorsal and caudal view of the heart from a 6 mm. pig embryo. X 21. 



foramina, and the sinus venosus opening into the right atrium (Fig. 109). This 

 opening is guarded by the right and left valves of the sinus venosus. Septa in- 

 co]jipletely separate the two atria and the two ventricles. In Fig. 109 the atrial 

 septum {septum primum) appears complete due to the plane of the section. 

 In Fig. 101, from a slightly smaller embryo, it is seen that the septum primum 



grows from the dorsal atrial waU 

 of the heart and does not yet 

 meet the endocardial cushions be- 

 tween the atrio-ventricular canals. 

 This opening between the atria is 

 known as the interatrial foramen. 

 Before it closes, another opening 

 appears in the septum, dorsal in 

 position. This is the foramen ovale 

 and persists during fetal hfe. In 

 Fig. 101 these two openings may be seen, as may also the dorsal and ventral 

 endocardial cushions which bound the atrio-ventricular foramina. The outer 

 mesothelial layer of the ventricles has become much thicker than that of the 

 atria. It forms the epicardium and the myocardium, the sponge-like meshes of 

 which are now being developed. 



Bulbus conli 



R. ventricle 



Interventricular 



foratiini 



Foramsn ovale 

 ■\\\'aU of I. atrium 



I liter atrial foramen 

 /■Endocardial cushions 



]]'all of I. ventricle 



Fig. 101. — Dissection of a, 5.5 mm. pig's heart 

 from the left side, showing the septum primum and 

 the interatrial and oval foramina. X 14. 



