342 OUTLINES OF CHORDATE DEVELOPMENT 



Car com 



left ventricle alone, so that the blood from this side of the heart 

 is now carried by the third arches to the carotids, and by the 

 right systemic or fourth arch to the general dorsal aorta. 



The fifth aortic arches having already disappeared, only the 

 sixth is left connecting now with the right ventricle. The 

 sixth arches ultimately form the roots of the pulmonary arteries, 



but throughout embryonic 

 life they remain, on each 

 side connected with the 

 roots of the definitive dor- 

 sal aorta. The true pul- 

 monary arteries are small 

 vessels passing backward 

 from the upper ends of the 

 sixth or pulmonary arches 

 (Fig. 134). That part of 

 each arch between the 

 origin of the pulmonary 

 artery and the dorsal aorta 

 is known as the ductiis 

 Botalli, and shortly after 

 hatching the two ductus 

 Botalli become closed as 

 strands of connective tissue, 

 turning the whole of the 

 blood stream of these 



FIG. 135. The heart and aortic arches j, rr ,l-, p<5 -nino- frnrn the 



of a chick embryo the latter part of the *} S >. 



sixth day. From a dissection. From right side of the heart, into 



Lillie (Development of the Chick) after ,, , 



Sabin. Au. Auricles; Car. com., common the pulmonary arteries, 



carotid artery; S'cl.d., S.cl.s., primary and Thereupon the dorsal re- 

 secondary subclavian arteries; 3, 4, 6, third 



(carotid), fourth (systemic), and sixth mainder of the left lateral 



(pulmonary) aortic arches. i i -i- i 



dorsal aorta disappears and 



leaves the dorsal aorta connected with the heart onh* by the 

 right systemic arch (Fig. 135). 



Certain branches of the aortic arches and dorsal aorta deserve 

 a special word. The dorsal aorta gives off segmental branches 

 between the somites, known as the segmental arteries (Fig. 109, 



