336 Dr Duckworth, A critical description of three cases 
enlarged so much as to have become the main path for the blood 
on its way to the placenta. Thus the origin of the large ventral 
artery becomes intelligible. 
(c) These changes were bilateral, but the two symmetrical 
ventral vessels thus formed approached one another and fused 
along part of their length. The median position of the ventral 
aorta is thus explained. 
Fig. 15. Sketch (based on a drawing by Professor A. Eobinson) of the arteries 
of the abdomen at an early stage in tbeir normal development. 
Should this interpretation be correct, it follows that the large 
ventral aorta has a complex nature. Near its origin it belongs 
to the series of segmental splanchnic vessels, of which the 
superior mesenteric artery and its branches are the most clearly 
recognisable representatives. In its lower parts, however, this 
ventral artery represents a persistent vitelline branch of the 
hypogastric artery continuous posteriorly with that part of the 
caudal arch termed the primitive posterior ventral aorta, from 
which the limb artery and the hypogastric artery are given off. 
In the intermediate portion of its course, the artery thus consti- 
tutes a channel of communication between a segmental splanchnic 
vessel and the primitive ventral aorta. Such a connexion has 
been observed in embryonic rodents (cf. Professor Young and 
Dr Robinson, Studies in Anatomy, from the Anatomical Depart- 
ment of the Owens College, Volume II, 1900, Plate I B, Fig. 18). 
