THE ARTERIES. 307 



or dorsal part which connects the root of the pulmonary artery 

 with the aorta. This distal portion is spoken of as the ductus 

 Botalli, or ductus arteriosus ; and so long as it remains open the 

 blood from the right ventricle can avoid the lung circulation, 

 and pass to the aorta direct. Shortly after the hatching of the 

 chick, however, the ductus Botalli shrivels up, and its cavity 

 becomes obliterated ; from this time the fifth arch communi- 

 cates with the lungs alone, and all the blood from the right 

 ventricle must pass through the pulmonary capillaries. 



b. The Dorsal Aortse and their branches. The two dorsal 

 aortse are at first separate along their whole length, running 

 parallel to the notochord and some little distance from the 

 median plane. Each aorta is. from the first, continuous along its 

 outer side at several places with the vascular reticulum of the 

 area pellucida and area vasculosa. By coalescence of the vessels 

 of the reticulum at one place, with shrinking and disappearance 

 of the reticulum in front of and behind this spot, the definite 

 vitelline artery of each side is formed (Figs. 113 and 128, AV) : 

 this is a large vessel running outwards from the aorta, between 

 the splanchnopleuric mesoblast and the hypoblast, and passing out 

 beyond the embryo to open distally into the vascular reticulum 

 of the area vasculosa. 



Before the end of the second day the two aorta3 have met and 

 fused for a short distance along the middle part of their course, 

 separating again towards their hinder ends, and giving off at 

 intervals along their length small arteries to supply the various 

 parts of the body. 



By the fourth day the union of the two aortae has extended 

 much further back than before, and involves the part from which 

 the vitelline arteries arise. The two vitelline arteries have 

 themselves coalesced at their proximal ends, and now spring 

 from the aorta as a single trunk, which divides almost at once 

 into the right and left vitelline arteries, of which the left one is 

 much the larger. 



The allantoic, or, as they are often called, umbilical arteries 

 (Fig. 128, AA), arise from the aortas just beyond their point of 

 bifurcation, and run outwards to the allantois. The left allantoic 

 artery is usually the larger of the two from the first, and becomes 

 ultimately the sole one, the right allantoic artery disappearing. 



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