250 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



of 3 to 4 mm., the bulbo- ventricular loop shifts its position until its base is directed 

 caudad and ventrad (Fig. 257 B). At the same time the sinus venosus is brought 

 dorsal to the atrium, which in turn is cranial with relation to the bulbo-ventricular 



Fig. 257. — A, Heart of human embryo of 2. IS mm.: a, Bulbus cordis; 6, primitive ventricle; c, 

 atrial portion. B, Heart of human embryo of about 3 mm.: a, Bulbus cordis; b, atrial portion (behind); 

 c, primitive ventricle (in front). Ventral views (His). 



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 the left 



^^y.t^-'m^ 



Fig. 258. — A, Heart of human embryo of about 4.3 mm.: u, Atrium; b, portion of atrium corre- 

 sponding to auricular appendage; c, bulbus cordis; d, atrial canal; c, primitive ventricle. B, Heart of 

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

 groove; c, right ventricle;/, left ventricle. Ventral views (His),. 



umbihcal 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. 



