2 S 8 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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 the left 

 umbilical vein is shifted to the right side through the liver. As a result the en- 

 larged right horn of the sinus opens into the right dorsal wall of the atrium through 

 a longitudinally oval foramen, which is guarded on the right by a vertical fold. 



. f 



FIG. 250. A, heart of human embryo of 2.15 mm. (His): a, bulbus cordis; b, primitive ventricle; 

 c, atrial portion. B, heart of human embryo of about 3 mm. (His): a, bulbus cordis; b, atrial portion 

 (behind); c, primitive ventricle (in front). 



FIG. 25i> A, heart of human embryo of about 4.3 mm. (His): a, atrium; b, portion of atrium 

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

 heart of human embryo of about the fifth week (His): a, left atrium; b, right atrium; c, bulbus cordis; 

 d, interventricular groove; e, right ventricle; /, left ventricle. 



This fold, which projects into the atrium, is the right valve of the sinus venosus. 

 Later, a smaller fold forms the left valve of the sinus venosus (Fig. 253, -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. 2$i,A, 

 B) with the distal portion of the bulb between them. The deep external groove 



