578 



EMBRYOLOGY 



from the yolk-sac ; they are the two oldest and largest venous trunks 

 of the body, but they become inconspicuous in the same ratio as the 

 yolk-sac shrinks to an umbilical vesicle. They run close together 

 along the intestinal tube, and come to lie at the sides of the duodenum 

 and stomach, where they are united to each other by transverse 

 anastomoses even at a very eaily period. 



The navel veins (venae umbilicales) are also originally double. At 

 'first very small, they subsequently become, in contrast with the 

 vitelline veins, more and more voluminous, as the placenta, from 



Fig. 320. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo 

 R, His), to illustrate the development of the pericardio-thoracic cavity and the diaphragm, 

 after His. 



<tb, Aortic bulb ; brh, thoracic cavity (recessus parietalis, His) ; hh, pericardial cavity ; <lc, ductus 

 Cuvieri ; dv, vitelline vein (v. omphalomesenterica) ; nv, umbilical vein ; vca, cardinal 

 vein ; vj, jugular vein ; Ig, lung; z + l, fundament of the diaphragm and the liver; uk, 

 lower jaw. 



which they convey the blood back to the body of the embryo, is 

 further developed. At the time of their first appearance the umbilical 

 veins are found to be imbedded in the lateral wall of the abdomen 

 (fig. 313 Vu), in which they make their way to the septum trans- 

 versum and the sinus venosus (sr). 



The inferior vena cava (fig. 321 A ci) is established later than any 

 of these paired trunks. It makes its appearance as an inconspicuous, 

 from the beginning unpaired, vessel (in the Rabbit on the twelfth 

 day, HOCHSTETTER) on the right side of the aorta in the tissue 

 between the two primitive kidneys ; caudal wards it is connected py 



