AORTIC-ARCH SYSTEM IX THE HUMAN EMBRYO. 107 



ing of this angle, the origin of the right pulmonary is carried ventrally, and near its 

 origin the vessel becomes somewhat curved about the proximal end of the aortic arch. 

 The segment of the main pulmonary vessel between the origin of the right and left 

 pulmonary arteries shifts away from the heart and toward the aorta. It is not 

 certain to what degree this is due to inequality in the growth of the segments 

 proximal and distal to the vessels and to what degree it is a matter of movement 

 of the origins of the two vessels in the wall of the main stem. Several things, 

 however, point to its being due chiefly to the former cause. During the period of 

 rapid descent, the points of origin of the two pulmonary arteries rapidly approach 

 each other. This must be due to the movement of one or both through the sub- 

 stance of the main vessel. They meet before the 40-mm. stage. The manner in 

 which they come together does not favor the view that they have become wrapped 

 about the pulmonary stem by means of its rotation and, by fusion with it, gradually 

 approach each other at their points of origin. After coming together the arteries 

 are designated bj^ their adult terminology right and left branches of the pulmonary 

 artery. The main stem proximal to them is the pulmonary artery, and the channel 

 from the origin of the left pulmonary to the dorsal aorta is the ductus arteriosus. 



Subclavian Artery. 



In their development the subclavian and vertebral arteries show to an unusual 

 degree the capacity of the blood-stream to take over and mold into a unit a number 

 of previous channels. The subclavian artery can first be recognized at the time the 

 forelimb-bud is but a slight elevation, after the completion of the fourth arch and 

 before the pulmonan r arch is complete. It is at this time a slender channel lying 

 in the sixth cervical intersegmental space and coming off from a sac-like projection 

 of the aorta winch later develops into the stem of a segmental artery. There are 

 similar vessels in adjacent intersegmental spaces which usually end in a plexus 

 before reaching the limb-bud. It seems probable, as far as can be determined from 

 the study of sections, that a second artery may occasionally extend into the limb- 

 bud, but this was not possible to ascertain with certainty in the absence of injection 

 of in toto preparations. Since the vertebral artery arises from the subclavian, it 

 usually enters the transverse process of the sixth cervical vertebra. The situation 

 of the origin of the subclavian and vertebral a segment more cranial than usual is of 

 much more frequent occurrence in the embryo than in the adult. There is evidence 

 of a regulation, in a sense, back to the usual type. 



The subclavian first passes to the limb-bud on the dorsal side of the brachial 

 plexus, but later it is inclosed by an outgrowth of neurons over its dorsal surface. 

 In a 14-mm. embryo one can distinguish radial, ulnar, interosseous, and some digital 

 arteries, as well as different nerves of the limb. The subclavian soon incorporates, 

 as a part of itself, the stem of the segmental from winch it arose. At this time its 

 segmental part comes off the unpaired aorta. As the aorta shifts caudally, the 

 subclavian is moored by the vertebrals, and then other branches move each onto 

 the corresponding paired aorta. At the time of rapid descent the right paired 

 aorta is interrupted just where it goes over into the unpaired aorta and the subcla- 



