506 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 
SAUROPSIDA AND 
FISHES. AMPHIBIANS. MAMMALS. 
(a) Mandibular aortic} Aborts, or is not} At most merely em- 
arch usually aborts; | developed. bryonic. 
there isa persistent 
trace in Elasmo- 
branchs (spiracular 
artery). 
(6) Hyoid aortic arch | Aborts. At most merely em- 
aborts, or is rudi- bryonic. 
mentary. 
(c) Ist branchial. Carotid. Carotid. 
(d@) 2nd branchial. Systemic arches, | Systemic. Onlythe right | 
unite to form} persists in Birds; only 
dorsal aorta, the left in Mammals. 
(e) 3rd branchial. Rudimentary or | Disappears. 
disappears in 
most forms. 
(f) 4th branchial (gives | Pulmonary. The pulmonary. 
off artery to “lung” 
of Dipnoi). 
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, 
and not directly to the heart. 
