300 THE DEVELOPMENT OF THE CHICK 
have entirely disappeared. The surviving arches are the third 
or carotid arch, the fourth or aortic arch, and the sixth or pul- 
monary arch. Up to this time the development is symmetrical 
on both sides of the body. 
During the fifth and = sixth 
days the two sides become 
asymmetrical, the fourth arch 
becoming reduced on the left 
side of the body and enlarged 
on the right. Fig. 207 shows 
the condition on the two sides 
Fig. 206. — Camera sketch of the aortic of the body on the sixth day. 
arches of the left side of a chick em- [Tf the fourth arch of the two 
bryo 44 days old. From an injected sides be compared it will be 
specimen. (After Locy.) : ; 
Abbreviations as in Fig. 205. seen that the left one is re- 
duced to a very narrow rudi- 
ment which has lost its connection with the bulbus arteriosus, 
while on the right side it is well developed. Another important 
change illustrated in the same figure is the reduction of the dorsal 
aorta between the upper ends of the carotid and aortic arches to 
& narrow connection. Two factors co-operate in the diminution 
Car ext 
Car com 
A 
Kia. 207. — Reconstruction of the aortic arches of a 6-day 
chick embryo from a series of sagittal sections. 
A. Left side. 
B. Right side. 
Car. com., Common earotid. Car. ext., External carotid. 
Car. int., Internal carotid. D.a., Duetus arteriosus. 
3, 4, and 6, Third, fourth, and sixth aortie arches. 
and gradual disappearance of this part of the primitive dorsal 
aorta, viz., the elongation of the neck and the reduction of the 
blood current. It will be seen that relatively little circulation 
is possible in this section, because the current up the carotid 
