VIII 



THE BIRD WITHIN THE EGG 



283 



By the end of the fifth day the ventricle of the heart 

 has divided into right and left chambers. On the 

 seventh day the artery leading from it follows suit and 

 divides into right and left branches, which are quite 

 separate. And of these two arteries, that from the 

 right chamber connects with the sixth pair of arches, 

 while that from the left connects with the third 

 and fourth. Meanwhile the lungs have come into 

 existence, being in their origin pouches formed in 

 the gullet. The sixth pair of arches open a connection 



Fig. 70.— (After Foster and Balfour.) Diagrams illustrating arterial circulation 

 (a) on third day— the first three pairs of arches, Nos. 4 and 6 not yet formed, No. 5 

 not represented ; (b) on the fifth or sixth day. The first, second, and fifth pairs 

 have been obliterated. 



with them, and here we have the final arrangement, 

 the artery from the right side of the heart leading to 

 these arches, and, so, onward to the lungs. Mean- 

 while the third pair of arches has been growing weaker, 

 and the fourth stronger. Finally, only one of the fourth 

 pair is left, that on the right side ; it is the great aorta, 

 which may be seen curving to the right, one of the 

 three single survivors out of five pairs, the other two 

 being the pulmonary arteries, which lead to the left 

 and right lungs and represent the sixth pair. Occa- 



