814 THE CHICK. 



to the head and fore-limbs ; while the right ventricle forces its 

 venous blood through the pulmonary trunk and the fifth pair of 

 aortic arches into the dorsal aorta, from which part goes to 

 supply the body and hind limbs of the embryo, and part, in the 

 earlier stages by far the larger part, passes out along the vitelline 

 and allantoic arteries to the yolk-sac and allantois, where it 

 takes up nutriment and oxygen. 



The enormously disproportionate size of the head and anterior 

 part of the embryo and the stunted condition of the hinder 

 part during the earlier stages are to be ascribed, at any rate in 

 part, to this arterial supply of the anterior half as contrasted 

 with the venous supply of the posterior half of the embryo. 



c. The changes in the circulation on hatching. The changes 

 which occur in the blood-vessels at or about the time of hatching 

 of the chick are comparatively slight, but suffice to convert the 

 circulation into that of the adult bird. The more important of 

 these changes are the following : — 



(i) Closure of the ductus arteriosus of each side : i.e. oblite- 

 ration of the distal parts of the fifth pair of aortic arches, beyond 

 the points of origin of the pulmonary arteries (of. Fig. 128, as). 

 The effect of this change is that the blood from the right ventricle 

 can no longer pass into the aorta, but is sent entirely, through 

 the pulmonary arteries, to the lungs. 



(ii) Obliteration of the vitelline and allantoic veins. This is 

 a necessary consequence of the complete absorption of the yolk- 

 sac, and the casting off of the allantois. Its effect is to reduce the 

 blood supply of the liver to the venous blood brought by the 

 mesenteric vein, or, as it may now be called, the hepatic portal 

 vein. 



(iii) Closure of the ductus venosus. The effect of this change 

 is to render it impossible for the blood brought to the liver to 

 pass straight through it to the heart. All the blood entering the 

 liver by the portal vein must now pass through the capillaries of 

 the liver in order to get to the posterior vena cava, and so to the 

 heart. The short cut by which the liver capillaries could pre- 

 viously be avoided is now stopped. 



