DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM 



763 



aortic stem is brought directly ventral to and applied to the auricular canal. By the upgrowth 

 of the ventricular septum the bulbus cordis is in great measure separated from the left ventricle, 

 but remains an integral part of the right ventricle, of which it forms the infundibulum. 



The primitive aortic stem is divided by the aortic septum (Fig. 541). This makes its appear- 

 ance as two ridge-like thickenings which project into the lumen of the tube; these increase in 

 size, and ultimately meet and fuse to form the septum, and thus the primitive aortic stem is 

 divided into the pulmonary artery and the aorta. The aortic septum takes a spiral course 



Aorta. 



Aorta. 



Aorta. 



Pulmonary 

 artery. 



Pulmonary 

 artery. 



FIG. 541. Transverse sections through the aortic bulb to show the growth of the aortic septum, 

 section is on the left, the highest on the right of the figure. (After His.) 



The lowest 



toward the proximal end of the stem, so that the two vessels lie side by side above, but near the 

 heart the pulmonary artery is in front of the aorta (Fig. 418). The septum grows down into the 

 ventricle as an oblique partition, which ultimately blends with the ventricular septum in such a 

 way as to bring the bulbus cordis into communication with the pulmonary artery, and through 

 the latter with the sixth pair of aortic arches; while the left ventricle is brought into continuity 

 with the aorta which communicates with the remaining aortic arches. 



Septum sj)tiriitm 



Opening of sinus venosus 

 Left venous valve 



Septum primuiu 



Right venous 

 valve 



Spina vestibuli 



Posterior endocardial 

 cushion 



Auricular cana 



Septum inferius 

 FIG 542. Interior of dorsal half of heart from a human embryo 10 mm. long. (His.) 



The Valves of the Heart. The auriculoventricular valves are developed in relation to 

 the auricular canal. By the upward expansion of the bases of the ventricles the canal becomes 

 invaginated into the ventricular cavities. The invaginated margin forms the rudiments of the 

 lateral cusps of the auriculoventricular valves; the mesal or septal cusps of the valves are 

 developed as downward prolongations of the septum intermedium. The aortic and pulmonary 

 valves are formed from four endocardial thickenings an anterior, a posterior, and two lateral 

 which appear at the proximal end of the primitive aortic stem. As the aortic septum grows 

 downward it divides each of the lateral thickenings into two, thus giving rise to six thickenings 

 the rudiments of the semilunar valves three at the aortic and three at the pulmonary orifice. 



