414 



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 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. 



\ 



Fig. 7. Reconstruction of Aortic arches in Pig embryo of the 20th day. 

 X about 45. 



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 termination gives off 

 a posteriorly directed vessel which, though as yet small and broken 

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



Embryo of the twentieth day (number 2). This em- 

 bryo is nearly of the same age as the preceding but shows variations 



