408 Harriet Lehmann, 



the third arch. The portion of the aortic root below the fourth 

 aortic arch is slightly larger on the left than on the right side. A 

 pulmonary branch from the sixth arch has become established. It 

 is symmetrical on both sides. The subclavians arise from the aortic 

 trunk about Vo mm below the union of the aortic roots. 



To summarize the conditions in the embrj^o: the position of the 

 proximal portion of the vessel passing forward from the ventral end 

 of the third arch, corresponds to that of the ventral remnant of the 

 second arch present in the preceding embryo. A study of inter- 

 mediate stages would be necessary to determine whether more than 

 the proximal portion is to be considered as a remnant of the second 

 or first arch. The reduced calibre of the dorsal aorta between the 

 third and fourth arches indicates the first stage in the degeneration 

 in that part. The development of the pulmonary artery in this 

 embryo agrees with the descriptions of His, Zimmeemann and Bremee, 

 and not with that of Eathke. The subclavians arise from the 

 aortic trunk, but nearer the union of the aortic roots than in earlier 

 stages. 



General summary for rabbit embryos. The condition 

 of the aortic arches in the six stages of rabbit embryos may now 

 be summarized. 



The mandibular aortic arches are the first to appear and for 

 a short time are the only ones present. They occupy a position for 

 the most part near the center of the mandibular bar but toward 

 their extremities bend into the truncus arteriosus below, and the 

 aortic roots above. At this time the blood passes directly from the 

 heart, through the first pair of arches, to the dorsal aortae, which 

 are separate throughout their course. In an early stage the man- 

 dibular arch appears to consist of a series of rather large, loosely 

 connected sinuses, with slender trabeculae crossing its lumen at 

 intervals. 



By the time the second and third aortic arches are nearly 

 complete the walls of the first arch are continuous, and its lumen 

 is free from any reticulated structure. When the second arch has 

 reached its maximum size, degeneration has begun in the middle 

 portion of the first arch so that, in this region, only a series of 

 disconnected sinuses remains. A short dorsal remnant of the first 

 arch is present in the tenth day, but no corresponding vessel was 

 observed in later stages. A larger ventral remnant persists until 

 after all the arches are formed, but I was unable to determine 



