360 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- Jf ^^g fourth arch of the two 



bryo U days old. From an injected g-^j^g ^^ compared it will be 



^^A^uu"^^"-' ^' ^^ • °S^ OAK seen that the left one is re- 



Abbreviations as m h ig. 205. 



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 

 a narrow connection. Two factors co-operate in the diminution 



Carext, 



I 

 Cdr.com. 



Fig. 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 carotid. Car. ext., External carotid. 

 Car. int., Internal carotid. D. a.. Ductus arteriosus. 

 3, 4, and 6, Third, fourth, and sixth aortic 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 



