or posterior chamber of the primi- 
tive heart; after a time it grows 
forwards out of the septum into 
the pericardium, and is absorbed 
into the auricular portion of the 
heart. 
The middle part of the anterior 
ventral aorta lies in the dorsal wall 
of the pericardium and _ projects 
forwards into its cavity. It lies 
close to its fellow of the opposite 
side, and as the two vessels rapidly 
enlarge their inner walls approach 
each other, and, fusing together, 
form a single median vessel which 
constitutes the primitive heart (the 
sinus venosus being excepted) in- 
cluding the bulbus arteriosus, the 
latter beimg afterwards developed 
into the roots of the pulmonary 
and aortic vessels. 
The anterior portion of the 
anterior ventral aorta is embedded 
in the tissues at the upper or 
anterior part of the pericardium, 
that is, at the root of the neck. 
At first it is connected with the 
primitive dorsal aorta by a single 
cephalic aortic arch, but after- 
wards three, and eventually four, 
additional arches connect it with 
the anterior part of the primitive 
dorsal aorta. As the neck lengthens 
this part of the anterior ventral 
aorta is elongated. 
. 
5 
) 
Primary 
caudal arches 
Umbilical vein 
Branches to yolk sac 
Vitelline artery 
Yolk sae 
DEVELOPMENT OF THE BLOOD VASCULAR SYSTEM. 
Dorsal aortee 
PE oe es sat _ fon es 
877 
rapidly enlarges and unites with its fellow of the opposite side to form the sinus venosus 
Cephalic aortic__ lst cephalic aortic arch 
arch ; F 
2nd cephalic aortic arch 
Anterior ventral 
aorta —— 38rd cephalic aortic arch 
4th cephalic aortic arch 
Primitive dorsal 
aorta — : : 
Vertebral artery 7 —5th cephalic aortic arch 
\ Bulbus arteriosus. 
Subclavian artery EF 
7 Veutricle 
Primitive jugular Heart 
vein 
Duct of Cuvier 
Auricle 
Sinus venosus J 
Cardinal vein 
Portal vein 
Umbilical vein 
Branches to_ 
alimentary canal 
Hypogastric 
artery 
Posterior ventral Placenta 
aorta 
Caudal aortic 
arch 
Fie. 598.—DtaGRAM OF THE PRIMITIVE BLOOD-VESSELS 
AFTER THE FORMATION OF THE HEART. 
For the greater part of its extent it remains separate from its fellow 
Post-costal 
anastomoses 
7th pair of 
segmental arteries 
— Ist pair of 
; ~yA| Segmental arteries 
lst cephalic aortic arch 
2nd cephalic aortic arch 
3rd cephalic aortic arch 
4th cephalic aortic arch 
5th cephalic aortic arch 
Bulbus arteriosus 
| Ventricle 
Auricle 
Sinus venosus 
Vitelline vein 
Fig. 599.—DIAGRAM OF THE PRIMITIVE BLOOD-VESSELS AFTER THE FORMATION OF THE HEART, BUT 
BEFORE ITS SUBDIVISION By SEPTA INTO AURICLES AND VENTRICLES. 
of the opposite side, and it takes part in the formation of the great vessels of the head 
and neck. Posteriorly, however, it unites with its fellow of the opposite side to form the 
