112 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



cal veins, the left being the larger. Between the veins is the extremity of the hepatic diver- 

 ticulum. The body wall is continued ventrad to form a short umbilical cord. 



Section at the Level of Origin of the Vitelline and Umbilical Arteries (Fig. 114). 

 As the posterior half of the embryo is curved in the form of a half circle, sections caudal 

 to the liver, like this one, pass through the lower end of the body at the level of the pos- 

 terior limb buds. Two sections of the embryo are thus seen in one, their ventral aspects 



Spinal cord 



Notochord 

 R. post, cardinal vein 



Dorsal aorta 

 R. subcardinal vein 



Mesentery 



Cephalic limb of intestine 

 R. umbilical vein 



Caudal limb of intestine 



R. umbilical vein 



Tail 



Lower limb bud 

 Mesonephric duel 



Dorsal aorta 



Spinal co 



Spinal nerve 



Post, cardinal vein 



Mesonephros 



L. subcardinal vein 



L. vilelline vein 



L. umbilical vein 



L. umbilical vein 

 Rectum 



Mesonephric tubule 

 Mesodermal segment 



FIG. 114. Transverse section of a 6 mm. pig embryo at the level of the origin of the vitelline 

 artery. The lower end of the section passes through the posterior limb buds. X 26.5. 



facing each other and connected by the lateral body wall. In the dorsal part of the sec- 

 t?on the mesonephroi are prominent, with large posterior cardinal veins lying dorsal to them. 

 The trunk of the vitelline artery takes origin ventrally from the aorta. It may be traced 

 into the mesentery, and through it into the wall of the yolk sac. On either side of the 

 vitelline artery are the subcardinal veins, the right being the larger. In the mesentery may 

 be seen two sections of the intestinal loop (the small intestine, being cut lengthwise, the large 

 intestine transversely), and also sections of the vitelline artery and veins. In the lateral 



