266 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



formation of the interventricular septum there arise in the aortic bulb longitudinal 

 thickenings, four in the distal half, two in the proximal half of the bulb. Of the four 



distal thickenings two, which we will number a and c, are larger than the other 



,ji tft**-0 / \ f - 

 thickenings b and d. Thickenings a and c, which distally occupy right and left 



positions in the bulb, meet, fuse and divide the bulb into a dorsally placed aorta 

 and ventrally placed pulmonary trunk (Fig. 259) . Traced proximally they pursue 

 a spiral course, a shifting from left to ventrad, and c from right to dorsad, and 

 becoming continuous with the proximal swellings. Thickenings b and d are also 

 prominent at one point proximally and when the bulb in this region is divided by 

 ingrowing connective tissue into the aorta and pulmonary artery, the aorta con- 

 tains the whole of thickenings & and half of a and c, while the pulmonary trunk 

 contains the whole of d- and half of a and c (Fig. 260). The three thickenings 

 now present in each vessel hollow out on their distal surfaces and eventually form 



FIG. 260. Scheme showing division of bulbus cordis and its thickenings into aorta and pulmonary 

 artery with their valves. The division begins in B, the lateral thickenings dividing respectively into 

 a, e, and c,f. Rotation from right to left shown in D (Heisler). 



the thin- walled semilunar valves (Fig. 260). The anlages of these valves are 

 prominent in embryos of 10 to 15 mm. as thick plump swellings projecting into 

 the lumina of the aorta and pulmonary artery. 



The two proximal bulbar swellings fuse and continue the spiral division of the 

 bulb toward the interventricular septum in such a way that the base of the pul- 

 monary trunk, now ventrad and to the right, opens into the right ventricle, while 

 the base of the aorta, now lying to the left and dorsad, opens into the left ven- 

 tricle close to the interventricular foramen through which the two ventricles 

 still communicate (Fig. 259, B}. 



Closure of the Interventricular Foramen. The interventricular foramen in 

 embryos of 15 to 1 6 mm. is bounded (i) by the interventricular septum; (2) by 

 the proximal septum of the bulb ; and (3) by the dorsal portion of the fused endo- 

 cardial cushions. Soon these structures are approximated, fuse and by the de- 



