100 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



ventral aorta. Viewed from the caudal and dorsal aspect (Fig. 100), 

 the sinus venosus is seen dorsal to the atria. It opens into the right atrium 

 and receives from the right and left sides the paired common cardinal veins. 

 These veins drain the blood from the body of the embryo. Caudally, 

 the sinus venosus receives the two 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 liver 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 

 atrio-ventricular foramina, and the sinus venosus opening into the right 

 atrium (Fig. 109). This opening is guarded by the right and left valves of 

 Septa incompletely separate the two atria and the two 

 ventricles. In Fig. 109 the 



"Foramen male 



of I. atrium 



the sinus venosus. 



Bulbus cordis, 



R. ventricle 



Intenentrifular, 



foramen 



'Interatrial foramen 

 Endocardia! cushions 



Wall 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. 



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 wall of the heart 

 and does not yet meet the en- 

 docardial cushions between 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 life. In Fig. i o i these two 

 openings may be seen, as may also the dorsal and ventral endocardial 

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

 thelial 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. These begin with the ventral aorta, which takes origin 

 from the bulbus cordis. From the ventral aorta are given off pairs of 

 aortic arches. These run dorsad in the five branchial arches (Figs. 104 and 

 105) and join the paired 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 of the ventral aorta just caudal to the median 

 thyreoid gland. The fourth aortic arch is the largest. From the appar- 

 ent fifth arch small pulmonary arteries are developing. There is evidence 

 that this pulmonary arch is really the sixth in the series, the fifth having 

 been suppressed in development (cf. Fig. 272 B). Cranial to the first 



