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 obHteration 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 onlv a small 



A B. 



Fig. 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 obviouslv in connection with the svstemic 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 

 long 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, 



