374 THE DEVELOPMENT OF THE CHICK 
wall, inasmuch as direct connection with the veins of the allantois 
is not yet established. The meatus venosus receives the omphalo- 
mesenteric veins, and the blood of the allantois by way of the 
subintestinal vein (the latter arrangement of very brief duration). 
Thus at this time all of the blood is mixed together in the 
sinus venosus, viz., that re- 
ceived through the ducts of 
Cuvier, presumably venous, 
and that received through 
the meatus venosus, pre- 
sumably arterial, owing to its 
circulation in the superficial 
vascular network of the yolk- 
sac. Apparently there is no 
arrangement for separation 
or discrimination in the re- 
distribution of the blood. 
But on the other hand it 
should be noted that most 
ric. 215. — Region of the bifureation of of the blood comes from the 
the post-cava in the adult fowl. Ven- 
tral view. (After Miller). 
A.m.s.(A. 0.m.), Omphalomesenteric ; ek 
artery. A.i.s., Left internal iliac artery. of the embryo at this time; 
Le i. Vena cava inferior. V. 1. ¢. d.. ond that the blood of the 
Right common iliac vein. V.i.e.d., Right ; 
external iliac vein. V.i.i.d., Right inter- embryo itself cannot be 
nal iliac vein. YV.i. Ll. s., Left vena in- hiplaree . 
' ; ughly venous owing to th 
tervertebralis lumbalis.  V. sr. s., Left aa s . ee the 
suprarenal vein. Vv. g., Genital veins. shortness of the circuit and 
Vy.r.m., Great renal veins. 
yolk-sac, owing to the slight 
development of the vessels 
the delicate nature of the 
embryonic tissues, which, no doubt, permit direct access of oxygen. 
On the sixth day the embryonic circulation enters on a second 
phase, owing to the changes in the structure of the heart and 
arrangement of the vessels described in detail in the preceding 
part of this chapter. 
On the eighth day the circulation is as follows: The right 
and left ventricles are completely separate, and the former 
pumps the blood into the pulmonary trunk, the latter into the 
aortic trunk. The carotid arteries arise from the base of the 
aortic arch and convey the blood to the head, and also, by way 
of the subelavians, to the walls of the thorax and to the wing. 
The left aortic arch has disappeared, and the right arch is con- 
