THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 123 



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 

 sixth (apparent fifth) pair of aortic arches. Proximally the bulb is 

 undivided. The interventricular septum is complete except for the inter- 

 ventricular 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 interventricular septum, 

 while the right extends to the endocardial cushion. These folds eventually 

 fuse and the partition of the ventricular portion of the heart is completed. 



Sept. II R. atrium 



\ / Sept. I 

 Left valve of sinus venosus. iffifc ^Foramen male 



Right valve of sinus venosus^ )&Sg^j% /^\^ Aorta 

 Inferior vena cava ., 



Simis venosus 

 Tricuspid valve 



ulmonary artery 

 Interventricular foramen 



R. ventricle 



FIG. 125. Heart of 12 mm. embryo, dissected from the right side. 



The endocardium at the atrio-ventricular foramina is already undermined 

 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 

 disappeared. Cranial to the third arch, the ventral aortae become the 

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

 descending aortas constitute the internal carotid arteries. The ventral 

 aortae between the third and fourth aortic arches persist as the common 

 carotid arteries. The descending 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 caudally as the 

 median dorsal aorta. The sixth aortic arches (cf. p. 100) are connected 

 with the pulmonary trunk, and from them arise small pulmonary arteries 

 to the lungs. Dorsal inter segmental arteries arise, six pairs from the de- 



