424 Harriet Lehmann, 



and the pulmonic trunk now form a continuous large A'^essel joined 

 dorsally to the left aortic root. 



The subclavian artery arises from a common stem with a seg- 

 mental artery, about -/g mm cephalad to the union of the aortic 

 roots, and then bends outward passing to the fore-limb. The ver- 

 tebral artery now arises from the same stem, passing outward and 

 forward. 



To summarize, the carotid stem and its branches are now con- 

 nected with the more posterior portion of the arterial system only 

 by the union of the common carotid with the fourth arch. 



This obliteration of the dorsal aorta between the third and 

 fourth arches, begins first at the anterior end, and is inaugurated 

 by thickening of the walls, as Tandler has described for vessels 

 disappearing late in the history of the aortic arches. 



The right sixth arch appears to be degenerating in a similar 

 manner, and while still complete shows no connection with the 

 pulmonary artery. The decrease in size of the right aortic root 

 between its union with that of the left side, and the subclavian 

 artery foreshadows the eventual disappearance of that part of the 

 aortic system. 



The fourth arches have already undergone considerable modifi- 

 cation, both with relation to the other vessels and in their actual 

 shape. In the earlier stages the two fourth arches unite in the 

 same plane as their union with the common carotids, while in this 

 embryo they meet considerably nearer the heart, thus giving either 

 vessel a more arched appearance. 



General summary for the series of pig embryos. 

 Six pairs of aortic arches are formed in the pig embryo, and all 

 are established by the close of the twenty-first day of development. 

 Of these, the first two are usually degenerated to a large extent 

 before the remaining four pairs reach their maximum development. 

 The first two arches may persist, however, for some time after the 

 third, fourth and sixth arches are formed, as nearly complete vessels, 

 but greatly reduced in calibre. These arches degenerate in a manner 

 similar to that described for the rabbit. Small dorsal rudiments of 

 both the first and second arches were found in each stage observed. 

 The persistent remnant in the hyoid arch is usually somewhat larger 

 than that in the mandibular. In each of the first five stages ob- 

 served there is present only a very short branch passing forward 

 from the ventral end of the third arch. I am unable to say cer- 



