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 
a b 
Fic. 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 ; (4) 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- 
