io6 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



i.Cornmon 

 cardinal Vein 



i.vl/ell/'ne vein 



Left ventricle 



R. Atrium 



R. Vi hi line vein 



R.Ventricle 



FIG. 95. Dorsal and caudal view of the heart from a 6 mm. 

 pig embryo. X 21. 



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, from the liver sinusoids, gut and from the yolk-sac. 



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



show the atria in communi- 

 corc/,'s cation with the ventricles 

 through the atrio-ventricular 

 canals (Fig. 104), and the 

 sinus venosus opening into 

 the right auricle. This open- 

 ing is guarded by the right 

 and left valves of the sinus 

 venosus. Septa incompletely 

 separate the two atria and 

 the two ventricles. In Fig. 

 104 the atrial septum (septum 

 primum) appears complete. 



In Fig. 96, from a slightly smaller embryo, it is seen that the septum primum 

 grows from the dorsal atrial wall of the heart and does not yet meet the 

 endocardial cushions between the atrio-ventricular canals. This opening 

 between the atria is known as the inter atrial foramen. Before it closes, 

 another opening appears in the septum, 

 dorsal in position. This is the foramen 

 ovale and persists during fetal life. In 

 Fig. 96 these two openings may be seen, 

 as may also the dorsal and ventral endo- 

 cardial cushions. 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. 



The arteries begin with the ventral aorta, which takes origin from the bulbus 

 cordis. From the ventral aorta are given off five pairs of aortic arches. These 

 run dorsad in the five branchial arches (Figs. 99 and 100) and join the paired 

 dorsal or descending aorta. The first and second pairs of aortic arches are very 

 small and take origin from the small common trunks formed by the bifurcation 



Bulbus cordis 



Foramen ovale 

 WallofLaJrium 

 Inter atrial foramen 

 fndocardial cushion}' 



Wallcf.Lventricle 



FIG. 96. Dissection of a 5.5 mm. 

 pig's heart from the left side, showing 

 the septum primum and two interatrial 

 foramina. X 14. 



