DUCTUS VENOSUS 

 VENTRICLE 



POSTERIOR CARDINAL VEIN 

 DORSAL AORTA 



ALLANTOIC VEIN 



Fig. 151. The liver. As we have seen in Chapters 12 and 13, the liver forms 

 in close relationship to the ductus venosus. This vessel forms by the fusion of 

 the vitelline veins just posterior to the sinus venosus. Endodermal outgrowths 

 form around the ductus venosus, which is surrounded by mesenchyme and sup- 

 ported by a dorsal mesentery. The liver finally completely envelops the ductus 

 venosus. Later the vitelline veins lose their connection with this vessel and 

 blood flows through the liver and is picked up by the hepatic veins. This is the 

 hepatic portal circulation. In this section the posterior cardinal veins may be 

 seen on either side of the dorsal aorta. A smaller vessel, the allantoic vein, may 

 also be present. At this level the ventricle of the heart is included. 



POSTERIOR CARDINAL VEIN 



VITELLINE VEIN 



Fig. 152. The limb buds. Just posterior to the heart the somatic mesoderm 

 thickens considerably to form the mesenchyme of the wing buds. The midgut at 

 this point is still open to the yolk and the endoderm is relatively undifferentiated. 

 The section passes through the vitelline veins, which bring blood from the 

 vascular area over the yolk to the ductus venosus and thence to the heart. A 

 dorsal aorta and two posterior cardinal veins are present. The posterior cardinal 

 veins are close to the developing mesonephros. The relation of the veins to the 

 mesonephros was discussed in Chapters 12 and 14. 



MESONEPHROS 



VITELLINE ARTERY 

 DORSAL AORTA 



Fig. 153. The somites and mesonephros. The somites continue their dif- 

 ferentiation into a compact layer of dermatome, a less conspicuous layer of 

 myotome, and a diffuse mass of sclerotome. The nephrotome has developed into 

 a mesonephric duct and mesonephric tubules. The duct is lateral to the tubules. 



