LATER DEVELOPMENT OF VASCULAR SYSTEM 361 
arch turns forward and that up the aortic arch turns backward, 
hence there is an intermediate region of stagnation, and here 
the obliteration occurs. 
On the eighth day the changes indicated on the sixth day 
are completed. The left aortic arch has entirely disappeared, 
and the connection between the upper ends of the carotid and 
aortic arches is entirely lost on both sides (Fig. 208), though lines 
of apparently degenerating cells can be seen between the two. 
On the other hand, the upper end of the pulmonary arch (duct 
of Botallus) is as strongly developed on both sides as_ the 
right aortic arch itself. The pulmonary artery proper is rela- 
tively very minute (Fig. 208), and it can transmit only a small 
C4, 
Fic. 208. — Reconstruction of the aortic arches of an 8-day embryo from 
a series of sagittal sections. 
A. Left side. 
B. Right side. 
A. o. m., Omphalomesenteric artery. Ao. A., Aortic (systemic) arch. 
Car., Carotid. D.a., Ductus arteriosus. d. Ao., Dorsal aorta. p. A., Pul- 
monary artery. S’cl., Subclavian artery. V., Valves of the pulmonary 
artery. 
quantity of blood; the principal function of the pulmonary arch 
is obviously in connection with the systemic circulation. In 
other words, both sides of the heart pump blood into the aorta 
during embryonic life; after hatching, the duct of Botallus be- 
comes occluded as already noted, and the pulmonary circulation 
is then fully established. 
The Carotid Arch. With the retreat of the heart into the 
thorax, the internal and external carotids become drawn out into 
lone vessels extending through the neck region. The internal 
carotids then become approximated beneath the vertebral centra. 
The stem of the external carotid forms an anastomosis with the 
internal carotid in the mandibular region, and then disappears, 
