421 



aorta in front of it, and appears to be continuous with the latter on 

 account of the marked decrease in the diameter of the dorsal aorta 

 between the third and fourth arches. This is especially noticable in 

 the region near its union with the third arch where its channel has 

 become greatly reduced. 



The left fourth arch in this embryo is large and of especial 

 interest on account of its connection with the complete fifth aortic 

 arch. A short distance from its union with the truncus arteriosus, 

 the fourth arch increases greatly in width, and there is given ofif from 

 its posterior side, near the middle of the arch, a smaller, but perfectly 

 distinct vessel, which bending slightly downward, follows along the 



,' I 



Mr- 



Fig. 12. Reconstruction of Aortic arches in Pig embryo of 21th daj\ X about 18, 

 c.e. external carotid, c.i. internal carotid, ex. common carotid. 



course of the fourth arch and joins the dorsal aorta immediately beneath 

 it. Just ventral to its union with the aortic root, there passes back- 

 wards from the rudimentary fifth vessel, a branch which hes close to 

 the dorsal aorta and joins the sixth arch immediately ventral to its 

 union with the dorsal aorta. This vessel, upon the left side, is mo- 

 derately large and well defined. A vessel of similar form and re- 

 lations, but somewhat smaller, is present upon the right side. Two 

 distinct branchial pouches are discernable between the fourth and sixth 

 arches upon both sides. 



