250 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



cranial with relation to the bulbo-ventricular loop, and the bulbar limb is 

 pressed against the ventral surface of the atrium and constricts it 

 (Fig. 258 A). 



In embryos of 4 to 5 mm. the right portion of the sinus venosus grows 

 more rapidly than the left, this being due to the fact that the blood flow of 



FIG. 258. A, Heart of human embryo of about 4.3 mm.: a, Atrium; b, portion of atrium 

 corresponding to auricular appendage; c, bulbus cordis; d, atrial canal; e, primitive ventricle. 

 B, Heart of human embryo of about 10 mm.: a, Left atrium; b, right atrium; c, bulbus cordis; 

 d, interventricular groove; e, right ventricle;/, left ventricle. Ventral views (His). 



the'left umbilical vein is shifted to the right side through the liver. As a 

 result, the enlarged right horn of the sinus opens into the right dorsal wall 

 of the atrium through a longitudinally oval foramen, guarded on the right 

 by a vertical fold. This fold which projects into the atrium, is the right 



Pulmonary artery Aorta 



Aorta- 



Bulbu 



R. ventricle 



Atrium 



Jf...L. ventricle 



FIG. 259. Diagrams to show the reduction of the bulbo-ventricular fold (represented by 

 diagonal lines) due to its retarded development. (Modified after Keith.) 



valve of the sinus venosus. Later, a smaller fold forms the left valve of the 

 sinus venosus (Fig. 260 B). The atrium is constricted dorsally by the gut, 

 ventrad by the bulbus. It therefore must enlarge laterally and in so doing 

 forms the right and left atria (Fig. 258 A, B) with the distal portion of the 



