5 o6 



STRUCTURE OF VERTEBRATA. 



dorsal aorta. This aorta gives off vessels to the body, while in embry- 

 onic life it sends important vitelline arteries to the yolk, and (in 

 Reptiles, Birds, and Mammals) equally important allantoic arteries 

 to the allantois. 



Returning to the arterial system of a fish, we must consider the 

 arches more carefully, and compare them with those of Sauropsida 

 and Mammals, where they are no longer connected with functional 

 gill-clefts, and also with those of Amphibians, where the complications 

 due to lungs, etc., begin (see the following Table). 



SUMMARY AS TO AORTIC ARCHES 



The important features in the development of the venous system are 

 as follows : 



(a) In the embryo the vitelline veins bring back blood from the 

 yolk-sac, at first directly to the heart, and later to the liver. 

 Into these veins, blood returned from the intestine is poured 

 in increasing quantity by other veins. In the adult these 

 persist to form the hepatic portal system, by means of which 

 blood from the stomach and intestine is carried to the liver, 

 nd not directly to the heart. 



