DEVELOPMENT OF THE VASCULAR SYSTEM 



509 



The sinus venosus is at first situated in the septum transversum (a layer of 

 mesoderm in which the liver and the central tendon of the diaphragm are devel- 

 oped) behind the primitive atrium, and is formed by the union of the vitelline 

 veins. The veins or ducts of Cuvier from the body of the embryo and the umbilical 

 veins from the placenta subsequently open into it (Fig. 463) . The sinus is at first 

 place transversely, and opens by a median aperture into the primitive atrium. 



Bulb us cordi 

 Ventricle 



Atrium 

 Sinus venosufi 



Vitelline veins 



FIG. 461. Diagram to illustrate the simple tubular 

 condition of the heart. (Drawn from Ecker-Ziegler 

 model.) 



FIG. 462. Heart of human embryo of about fourteen 

 days. (From model by His.) 



Soon, however, it assumes an oblique position, and becomes crescentic in form; its 

 right half or horn increases more rapidly than the left, and the opening into the 

 atrium now communicates with the right portion of the atrial cavity. The right 

 horn and transverse portion of the sinus ultimately become incorporated with and 

 form a part of the adult right atrium, the line of union between it and the auricula 

 being indicated in the interior of the atrium by a vertical crest, the crista terminalis 

 of His. The left horn, which ultimately receives only the left duct of Cuvier, 



Maxillary process 

 Stomodeum 

 Mandibular arch 



Bulbus cordis 



Ventricle 

 Duct of Cuvier 



Cardinal vein 



Atrium 



Bile-duct 



Umbilical vein 



FIG. 403. Heart of human embryo of about fifteen days. (Reconstruction by His.) 



persists as the coronary sinus (Fig. 464) . The vitelline and umbilical veins are soon 

 replaced by a single vessel, the inferior vena cava, and the three veins (inferior vena 

 cava and right and left Cuvierian ducts) open into the dorsal aspect of the atrium 

 by a common slit-like aperture (Fig. 465). The upper part of this aperture repre- 

 sents the opening of the permanent superior vena cava, the lower that of the inferior 

 vena cava, and the intermediate part the orifice of the coronary sinus. The slit- 



