LATER DEVELOPMENT OF VASCULAR SYSTEM 357 



ridges, and that the aortic division of the bulbus lies above the 

 pulmonary division, that is, next the bulbo-auricular ridge. 

 The left bulbus ridge is thus continuous with the interventricular 

 septum immediately beneath the foramen of the latter, and the 

 right bulbus ridge lies on the opposite side. 



The bulbus septum now becomes complete by fusion of the 

 right and left sides. The blood from the left ventricle is then 

 forced in each systole through the interventricular foramen and 

 along a groove in the right side of the cushion-septum into the 

 aortic trunk. This groove, however, is open to the right ven- 

 tricle also above the septum of the bulbus; but it is soon bridged 

 over by an extension of the cushion-septum along the bulbo- 

 auricular ridge as far as the right side of the septum of the bulbus; 

 in this way the space existing between the interventricular sep- 

 tum and the opening of the aorta is converted into a tube, and 

 thus the aorta is prolonged through the cushion-septum, and 

 by way of the interventricular foramen into the left ventricle. 



Fate of the Bulbus. The distal portion of the bulbus is con- 

 verted into the proximal parts of the aorta and pulmonary artery. 

 The part proximal to the semilunar valves is gradually incor- 

 porated into the ventricles, owing to extension of the ventricular 

 cavities into its wall, and subsequent disappearance of the inner 

 wall of the undermined part. 



The Sinus Venosus. (For earlier development see Chap. VI; 

 relation to septum transversum, Chap. XI.) 



In the course of development, the sinus venosus gradually 

 separates from the septum transversum, though always connected 

 with the latter by the vena cava inferior. In early stages (up to 

 about 24 somites) it is placed quite symmetrically behind the 

 atrium, and extends transversely to the entrance of the ducts of 

 Cuvier on each side. The sinu-auricular aperture is approximately 

 in the median line at first, so that the right and left divisions of 

 the sinus are nearly symmetrical. The condition of approximate 

 bilateral symmetry of the sinus is, however, rapidly changed 

 by shifting of the sinu-auricular aperture to the right side with 

 the outgrowth of the right auricle (24-36 somites); thus the left 

 horn of the sinus becomes elongated; moreover, the main expan- 

 sion of the sinus takes place in the region of the sinu-auricular 

 aperture, and thus the left horn appears relatively narrow in diam- 

 eter. The interauricular septum forms to the left of the sinu- 



