Embryouic history of the aortic arches in mammals. 411 



The third aortic arch is the largest of the aortic vessels present, 

 and in this embryo is larger than in any other stage observed. It 

 is a short tubular vessel of fairly regular outline, but its walls do 

 not present the thickened appearance characteristic of the dorsal 

 aorta of this stage. 



The fourth arch, although longer, is smaller in diameter than 

 the third, and upon either side in the region of its union with the 

 dorsal aorta a small cord of mesoderm passes through its channel. 



The most posterior arch in this embryo has been considered the 

 sixth both because of its great distance from the fourth, and because 

 a fifth arch usually appears at a considerably later period. This 

 vessel is complete in its ventral half, and at its blind terminus gives 

 off a posteriorly directed vessel which, thoug as yet small and broken 

 in its course, appears to be an element of the pulmonary artery. 

 Dorsally from this vessel a group of sinuses extends toward the 

 aortic root, and a distinct small branch joins the latter on its inner side. 



The first aortic arch in this embryo, from comparison with later 

 stages, has been regarded as in the process of degeneration. Its 

 connection with the dorsal aorta has persisted longest, and the 

 characteristic pocket-like sinuses noted for the rabbit appear about 

 the ventral end of this remnant. The sinuses near the anterior 

 prolongation of the truncus are similar to those observed in the 

 rabbit, and afford evidence that the ventral portion of the first arch 

 does not retain its connection with the ventral end of the second 

 arch. The anterior end of the truncus, however, still projects beyond 

 the ventral end of the second arch. 



The manner of disappearance of the second vessel is similar 

 to that described for the rabbit, the vessel becoming incomplete 

 first in its middle region. 



The band of tissue crossing the lumen of the fourth arch near 

 its union with the dorsal aorta, corresponds to the similar condition 

 noted in the rat embryo by Tandler and designated by him „Inseln". 

 It is further described in several of the other stages of the pig 

 embryos. The channel of the anterior end of the aortic roots in 

 this stage is crossed by ingrowths of the surrounding tissue, forming 

 islands as it were, in the blood stream. 



The condition of the sixth arch in this embryo indicates that 

 in the pig, the pulmonary artery starts to form before the pulmonary 

 arch is complete. The distance between the fourth and sixth arches 

 is noteworthy. 



