DEVELOPMENT OF THE VASCULAR SYSTEM. 103 
ress of the embryo. Special points will be considered, either in 
a separate chapter now, or deferred for treatment in the body 
of the work from time to time, as they seem to throw light 
upon the subjects under discussion. 
DEVELOPMENT OF THE VASCULAR SYSTEM IN VERTE- 
BRATES. 
This subject has been incidentally considered, but it is of 
such importance morphological, physiological, and pathological, 
as to deserve special treatment. 
In the earliest stages of the circulation of a vertebrate the 
arterial system is made up of a pair of arteries derived from the 
single bulbus arteriosus of the heart, which, after passing for- 
ward, bends round to the dorsal side of the pharynx, each giving 
off at right angles to the yelk-sac a vitelline artery; the aorta 
unite dorsally, then again separate and become lost in the pos- 
terior end of the embryo. The so-called arches of the aorta 
are large branches in the anterior end of the embryo derived 
from the aorta itself. 
The venous system corresponding to the above is composed 
of anterior and posterior pairs of longitudinal (cardinal) veins, 
the former (jugular, cardinal) uniting with the posterior to 
form a common trunk (ductus Cuviert) by which the venous 
blood is returned to the heart. The blood from the posterior 
part of the yelk-sac is collected by the vitelline veins, which 
terminate in the median sinus venosus. 
The Later Stages of the Fetal Circulation. Corresponding to 
the number of visceral arches five pairs of aortic arches arise; 
but they do not exist together, the first two having undergone 
more or less complete atrophy before the others appear. Figs. 
119, 120 convey an idea of how the permanent forms (indicated 
by darker shading) stand related to the entire system of vessels © 
in different groups of animals. Thus, in birds the right (fourth) 
aortic arch only remains in connection with the aorta, the left 
forming the subclavian artery, while the reverse occurs in mam- 
mals. The fifth arch (pulmonary) always supplies the lungs. 
The arrangement of the principal vessels in the bird, mam- 
mal, etc., is represented on page 104. In mammals the two 
primitive anterior abdominal (allantoic) veins develop early 
and unite in front with the vitelline; but the right allantoic 
vein and the right vitelline veins soon disappear, while the long 
