26 



WILLIAM A. LOCY AND OLOF LARSELL 



middle of the third day before the two hinder pairs of aortic arches 

 are formed. On the appearance of the fifth (sixth !) pair of aor- 

 tic arches the pulmonary arteries become connected with their 

 ventral ends." 



The spur from the sixth aortic arch arises before the arch is 

 completed and at its first appearance it is on the ventral part of 

 the arch near the truncus arteriosis. The sixth arch, in common 

 with the others, is formed by a dorsal moiety from the dorsal 

 aorta (fig. 5) , and a ventral moiety from the truncus arteriosus. 



w%N 



Pul.|. 



At. t" 

 VI \ 



ly - 



III 



I, VI -V- 



At.-|---\ 



VI Y-~J 



Pul. 

 At. 

 VI 





Pul. 



Pul. 

 "At. 

 VI 



Fig. oS Cross section of an embryo of the 96-hour stage to 

 rt of tlie jiiihiionarv artery arising from the sixth aortic arch. 



ow the (list; 



The ventral moiety is the longer and as it grows the relati\'e posi- 

 tion of the arterial spur changes. At the 4^ day stage the posi- 

 tion of the pulmonaiy arteiy is nearer the truncus arteriosus 

 than the dorsal aorta. On the second half of the sixth day (fig. 

 12) it emerges at about the middle point of the sixth arch. The 

 change in position is continued until, in later stages (fig. 22) the 

 base of the pulmonary artery is nearer the dorsal aorta. 



While the figures just described convey a good idea of the ap- 

 pearance of the pulmonary artery from surface views, the study 

 of injected specimens as transparencies gives an idea of the in- 

 ternal distribution of vascular loops. Such a specimen of five 

 days nine hours incubation is represented in figure 59 . This is 



