THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 



121 



a sagittal slit guarded by the right and left valves of the sinus venosus. The right 

 valve is the higher and its dorsal half is cut away. The valves were united 

 cephalad as the septum spurium. Between the left valve and the septum primum 

 the sickle-like fold of the septum secundum is forming; the fusion of these three 

 components gives rise later to the adult atrial septum. The aortic bulb is divided 

 distally into the aorta and the pulmonary artery, the latter connecting with the 

 fifth pair of aortic arches. Proximally the bulb is undivided. The interven- 

 tricular septum is complete except for the interventricular foramen which leads 

 from the left ventricle into the aortic side of the bulb. Of the bulbar swellings 

 which divide the bulb into aorta and pulmonary trunk, the left joins the inter- 

 ventricular septum, while the right extends to the endocardial cushion. These 

 folds eventually fuse and the partition of the ventricular portion of the heart is 



Sept. II R. atrium 



Left valve of sinin 

 Right valve of sinus vi nosii 

 Inferior vena ca 



nun ovale 

 — [ill la 



\-Pidmonary artery 

 I nterventricular foramen 



R. ventricle 



Fig. 125. — ^Heart of 12 mm. embryo dissected from the right side. 



completed. The endocardium at the atrio-ventricular foramina is already under- 

 mined to form the anlages of the tricuspid and bicuspid valves. From the caudal 

 wall of the left atrium there is given off a single pulmonary vein. 



The Arteries. — As seen in Fig. 123, the first two aortic arches have dis- 

 appeared. Cranial to the third arch, the ventral aortas become the external 

 carotids. The third aortic arches and the cephalic portions of the descending 

 aortae constitute the internal carotid arteries. The ventral aortae between the 

 third and fourth aortic arches persist as the common carotid arteries. The de- 

 scending aortae in the same region are slender and eventually atrophy. The 

 fourth aortic arch is largest and on the left side will form the aortic arch of the 

 adult. From the right fourth arch caudad, the right descending aorta is smaller 

 than the left. Opposite the eighth segment, the two aortae unite and continue 



