212 



THE SIXTH DAT. 



[chap. 



near the end of incubation, when the chick begins to breathe 

 the air in the air-chamber of the shell, and respiration is no 

 longer carried on by the allantois. Blood then ceases to flow 

 along the umbilical vessels ; they become obliterated. The 

 omphalo-mesaraic vein, which as the yolk becomes gradually 

 absorbed proportionately diminishes in size and importance, 

 comes to appear as a mere branch of the portal vein. The 

 ductus venosus becomes closed, remaining often as a mere 

 ligament ; and hence the whole of the blood coming through 

 the portal vein flows into the substance of the liver, and so 

 by the two hepatic veins into the vena cava (Fig. 64, HP). 



Previous to these changes one of the veins passing from 

 the rectum into the vena cava has given off a branch which 

 effects a junction with one of the mesenteric veins. This now 

 forms a somewhat conspicuous connecting branch between the 

 systems of the vena cava and the portal vein (Fig. 64, Co.V.). 



All three venae cavae now fall exclusively into the right 

 auricle, and by the closure of the foramen ovale the blood 



State of Arterial Circulation on the Fifth or Sixth Day. 



E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, ivf. A. 

 arteries to the Wolffian bodies. Ver. A. arteries given off between each 

 pair of vertebrae. Of. A. omphalo-mesaraic artery. UA. umbilical artery. 

 I A. iliac artery. 



