Embryonic history of the aortic arches in mammals. 423 



The fourth arches unite at a;, and thus it will be seen that the 

 common carotid stems branch from the upper portion of either 

 systemic arch. The carotid stem divides first into two branches, 

 the smaller one of which runs ventral to the other, and nearer the 

 median plane. The larger branch just below its bend about the 

 pharynx, gives off a small branch which soon breaks up into lacunae; 

 these extend toward, and appear to unite with the first branch of 

 the common carotid at c. Besides the lacunae above described in 

 connection with this vessel, several others are found joining it at 

 its base, and upon the inner side. They also extend downward for 

 a short distance about the main branch of the common carotid. 

 This branch bends about the pharynx and then passes forward 

 dorsal to it. This is the internal carotid. 



In this stage I also observed two branches from the internal 

 carotid corresponding in the position of their basal portions, to the 

 spurs from the dorsal aorta noted in earlier embrj^os, in the mandi- 

 bular and hyoid visceral arches. 



The dorsal aorta between the third and fourth arches has become 

 greatly attenuated and near the dorsal end of the third arch is no 

 longer a complete vessel, but its former course can be followed by 

 the presence of small lacunae and condensed groups of cells. The 

 portion of the blood-vessel which is still complete has a thread-like 

 channel, with thick walls. 



The diameter of the fourth arch of the right side is now 

 slightly less than one-half that of the left side, and behind the 

 arches, the right aortic root to the base of the subclavian artery, 

 is of but about one-fourth the diameter of that of the left side. The 

 changes in the right side make the fourth arch and the aortic root 

 appear continuous with the root of the subclavian. Back of this 

 vessel, the right aortic root is extremely attenuated for the remainder 

 of its course. 



The lumen of the right sixth arch is of greatly reduced size, 

 and the walls are much thickened. The vessel appears to be 

 degenerated too far to admit longer of free passage of the blood. 

 It is united with the large left sixth arch, above the base of the 

 single pulmonary artery. No branch from the right pulmonary arch 

 is present but small lacunae lie about and in connection with, the 

 upper part of the pulmonary artery. The latter now arises from 

 the right side of the left sixth arch. This single pulmonary artery 

 eventually divides into two branches. The left pulmonary arch 



