THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



distance. From each division there arises laterad three short trunks which 

 unite to form the single umbilical artery on each side. The middle vessel 

 is the largest and apparently becomes the common iliac artery. A pair of 

 short caudal arteries, much smaller in size, continue the descending aortae 

 into the tail region. 



The Veins. The vitelline veins, originally paired throughout, are now 

 represented distally by a single vessel, which, arising in the wall of the 

 yolk sac, enters the embryo and courses cephalad of the intestinal loop 

 (Fig. 102). Crossing to the left side of the intestine and ventral to it, it 



Spinal cord 



Anterior cardinal vein 



Cervical sinus 



Pericardial cavity 



R. common cardinal vein 



Post, cardinal vein 



Esophagus 



Large venous sinusoid of liver 



Anterior limb bud 



Inf. vena cave 



Post, cardinal vein 



Mesonephros (cut surface) 



R. subcardinal vein 



Venous sinusoid on dorsum of t 

 mesonephros 



Notochord 



Pharynx 



Trachea 



L. common 



cardinal vein 

 Lung 



Liver 

 Stomach (cut edge) 



Omental bursa 

 Mesogastrium 



'Mesonephros (cut surface) 



Capillary anastomosis between 



subcardinal veins 

 Vitelline artery in dorsal 



mesentery 

 Capillary anastomosis bet-ween 



subcardinal veins 



Venous sinusoid on dorsum of 

 mesonephros 



'Spinal cord 



FIG. 103. Reconstruction of the cardinal and subcardinal veins of a 6 mm. pig embryo, 

 showing the early development of the inferior vena cava (K. L. Vehe). X 22. In the small 

 orientation figure (cf. Fig. 105) the various planes are indicated by broken lines * *. 



is joined by the superior mesenteric vein which has developed in the mes- 

 entery of the intestinal loop. The trunk formed by the union of these two 

 vessels becomes the portal vein. It passes along the left side of the gut in 

 the mesentery. Opposite the origin of the dorsal pancreas it gives off a 

 small branch, a rudimentary continuation of the left vitelline vein, which 

 courses cephalad and in earlier stages connects with the sinusoids of the 

 liver. The portal vein then bends sharply to the right, dorsal to the duo- 



