PRIMITIVE VASCULAR SYSTEM AND FCETAL CIRCULATION. 59 
some doubt about their ultimate fate, but it is generally believed that their nuclei 
disappear, and that they are converted into non-nucleated corpuscles. 
Directly after the appearance of the blood islands in the vascular area of the 
yolk-sac, and just as the folding off of the embryo commences, two short tubular 
vessels appear in the splanchnic layer of the pericardial mesoderm. These vessels 
at once extend forwards and outwards into the extra-embryonic region where they 
become connected with the vessels of the vascular area; they also extend backwards 
in the body of the embryo beneath the protovertebral somites. In the majority of 
mammals they at first terminate behind, as in front, on the wall of the yolk-sac, 
but after a time the main stems appear to be continued along the allantoic stalk 
to the placenta, and to give off branches to the yolk-sac. It is probable that in the 
human embryo also, though this has not apparently been actually observed, these 
main stem vessels, the primitive aortie, end at first on the wall of the yolk-sac, but 
on the fourteenth day of intrauterine life, before the heart is formed, the two 
primitive stem-vessels pass backwards along the body-stalk to the chorion, their 
terminal branches entering the chorionic vilh. As they pass backwards the 
primitive aorte give off branches to the wall of the yolk-sac. Thus, at this period 
the vascular system of the human embryo consists 
of two longitudinal vessels which run parallel with 
each other, one on each side of the middle line, 
throughout the whole length of the embryo. They 
communicate anteriorly with the vessels on the 
yolk-sac, and terminate posteriorly in the chorion. 
When the circulation commences the blood flows 
from the anterior part of the vascular area into 
the anterior ends of the primitive aorte, and 
passes backwards through the embryo. Some of 
it is returned to the vascular area by the branches 
which are given off to the walls of the yolk-sac ; 
but the greater part is carried to the chorion, 
whence it returns by venous channels, the allantoic 
Cephalic 
aortic arch 
Anterior 
ventral aorta 
Primitive 
dorsal aorta 
-Vitelline vein 
Umbilical vein 
Splanchnie 
fy arteries to 
vascular area 
Vitelline artery 
veins, which have been developed in the mean- Sree 
time, to the anterior ends of the primitive aorte. tf } av Maer 
As the cephalic and caudal folds are developed } caudal arch 
: a | aa : \f Bre OED Hypogastric 
the anterior and posterior parts of the prinitive \ aclery 
aorte are carried into the ventral wall of the (SAL eau 
oe ois MP Fk ) Chorionic 
body of the embryo, and thus each primitive ANY ‘vessels 
vessel is divisible into three parts: (1) a dorsal 
part. tl ‘imitive dorsal t hick tends Fic. 45.—THE PRIMITIVE BLOOD- VESSELS 
par , Lhe primitive dorsal aorta, W yen extends OF THE EMBRYO. 
from the dorsal end of the mandibular arch to 
the cloaca, and runs beneath the protovertebral somites; (2) an anterior ventral 
part, situated in the dorsal wall of the pericardium and extending from the 
Bee icus to the ventral end of the mandibular arch; and (3) a posterior ventral 
part, which at first runs in the ventral wall at the side of the cloaca, and then 
turns backwards in the body-stalk to the placenta, but afterwards, when the 
posterior part of the ventral wall of the body is completed, it extends forwards 
from the pelvic region to the umbilical orifice, through which it passes to the 
umbilical cord. 
The three sections are united together by two arches—an anterior arch, the first 
cephalic aortic arch, which passes through the mandibular arch, and a posterior 
arch, the caudal arch, which lies at the side of the cloaca. , 
In a short time four additional communications are formed between the anterior 
ventral and the dorsal part of each primitive aorta; they are the second, third, 
fourth and fifth cephalic aortic arches, each of which hes in the substance of the 
corresponding visceral arch. 
As soon as the last cephalic aortic arch is developed the rudiments of the main 
vessels of the embryo are established ; and by a series of transformations, for a full 
account of which the chapter which deals with the Vascular System must be 
consulted, there are formed from the vessels which have been mentioned the heart, 
