3/2 THE DEVELOPMENT OF THE CHICK 
terior cardinals passes forward to the ducts of Cuvier without 
entering the mesonephric circulation. On the fifth and sixth 
days the cephalic ends of the posteardinals gradually dwindle 
and disappear (cf. Fig. 216); thus all of the blood entering the 
posteardinals must pass through the mesonephros to the sub- 
‘ardinals, which thus become efferent vessels of the mesonephros; 
and a complete renal-portal circulation is established. 
This form of circulation continues during the period of func- 
tional activity of the mesonephroi, and as the latter gradually 
atrophy, the portions of the subeardinals posterior to the anas- 
tomosis gradually disappear. A direct connection between the 
post- and subcardinals is then established on each side, by way 
of the great renal veins, which have in the meantime formed in 
connection with the development of the kidney (Fig. 214). 
The crural and ischiadic veins have, in the meantime, developed 
in connection with the formation of the hind limbs, as branches 
of the posteardinals. Thus the hinder portion of the latter be- 
comes transformed into the common iliac veins, and at the hinder 
end the posteardinals form an anastomosis (Tig. 214). 
IV. THe EMBRYONIC CIRCULATION 
On the fourth day the blood is driven into the roots of the 
dorsal aorta through three pairs of aortic arches, viz., the third 
or carotid, the fourth or aortic, and the sixth or pulmonary. The 
fifth pair of aortic arches is also functional for a time during this 
day, but soon disappears. The blood passing up the third or 
carotid arch is directed forward through the internal and external 
carotid arteries to the head; that passing up the fourth and 
sixth arches turns backwards to enter the dorsal aorta, so that 
there is an intermediate area of stagnation in the roots of the 
dorsal aorta between the carotid and aortic arches; though this 
is more or less problematical, the arrangement of the vessels ren- 
ders such a condition very probable. A very small proportion of 
the blood enters the rudimentary pulmonary arteries from the 
sixth arch. The blood in the dorsal aorta passes backwards and 
enters (1) the segmental arteries, (2) the omphalomesenteric 
arteries, (3) the (rudimentary) umbilical arteries, and behind 
the latter passes into the narrow continuation of the dorsal aorte, 
still separate in this region, known as the caudal arteries. 
The blood is returned to the heart through the sinus venosus 
