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 

 of the body on the sixth day. 

 If the fourth arch of the two 

 sides be compared it will be 

 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 

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



FIG. 206. Camera sketch of the aortic 

 arches of the left side of a chick em- 

 bryo 4 days old. From an injected 

 specimen. (After Locy.) 

 Abbreviations as in Fig. 205. 



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 



