258 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Origin of the Right and Left Ventricles. — In embryos of 5 to 6 mm. there 

 appears at the base of the primitive ventricular cavity a sagittally placed eleva- 

 tion, the interventricular septum (Fig. 260 B). It later grows cephalad and 

 dorsad toward the endocardial cushions, and forms an incomplete partition be- 

 tween the right and left ventricles, which still communicate through the persisting 

 interventricular foramen (Fig. 266 B). Corresponding to the internal attach- 

 ment of the septum there is formed externally the interventricular sulcus (Fig. 

 266 A) which marks the external Hne of separation between the large left ven- 

 tricle and the smaller right ventricle. 



Origin of Aorta and Pulmonary Artery from Bulbus. — Coincident with the 

 formation of the interventricular septum there arise in the aortic bulb (including 

 its distal truncus arteriosus) longitudinal thickenings, four in the distal half, two 

 in the proximal half. Of the four distal thickenings (Fig. 267), two, which may be 



Aorta 



Pulmonary artery 



Fig. 267. — Scheme showing division of bulbus cordis and its thickenings into aorta and pulmonary 

 artery with their valves. (Explanation in text.) 



designated a and c, are larger than the other thickenings, h and d. Thickenings 

 a and c, which distally occupy left and right positions in the bulb, meet, fuse, and 

 divide the bulb into a dorsally placed aorta and ventraUy placed pulmonary 

 trunk (Fig. 266). Traced proximally they pursue a clockwise, spiral course, a 

 shifting from left to ventral, and c from right to dorsal, both becoming con- 

 tinuous with the proximal swellings. Thickenings b and d are also prominent at 

 one point proximally; when the bulb in this region is divided by ingrowing con- 

 nective tissue into the aorta and pulmonary artery, the aorta contains the whole 

 of the thickenings b and half of a and c, while the pulmonary trunk contains the 

 whole of d and half of a and c (Fig. 267). Distally the three thickenings now pres- 

 ent in each vessel disappear, but proximally they enlarge, hollow out on their 

 distal surfaces, and eventually form the thin-walled semilunar valves (Fig. 267). 

 The anlages of these valves are prominent in embryos of 10 to 15 mm. as plump 

 swellings projecting into the lumina of the aorta and pulmonary artery. 



