AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 85 



elusions as held by Bremer and Fedorow as to the developmental relations of the 

 two arterial channels. Embryos in good histological condition showed the presence 

 of a pair of arteries extending backward from the aortic sac before the pulmonary 

 arch was complete (figs. 33 and 34). The arch was formed by the establishment 

 of a connection between this vessel and the dorsal aorta in the plexus lying caudal 

 to the caudal pharyngeal complex. Just after the arch has completed its channel, 

 its divisions proximal and distal to the pulmonary artery join at a marked angle. 

 As the arch increases in caliber this disappears in a continuous curve. 



The next important change in the pulmonary arterial vessels is the inter- 

 ruption of the right arch which has been already described. As the distal portion of 

 the arch degenerates, the angle between its proximal division and the right primi- 

 tive pulmonary artery is gradually lost, and the segment from now on functions 

 exclusively as the proximal end of the artery (fig. 17, c, d). The mechanism of 

 these changes has already been sufficiently discussed. The interruption of the 

 arch also similarly allows a straightening of the angle at the junction of the pul- 

 monary trunk and the left arch. Since the origin of the right pulmonary is at the 

 plane where these two territories are confluent, it is carried to the left, and the 

 artery is made to pass obliquely across the trachea and also to sink ventrally, so 

 that it is forced to curve slightly around the ascending limb of the aortic arch. 



In the further history of the pulmonaries we must distinguish clearly between 

 the earlier post-branchial phase and the period of rapid descent of heart and large 

 vessels. Measurements on models of a series of 11 embryos showed that the 

 distance between the origin of the two vessels remained nearly constant and equal 

 to its precursor (the proximal segment of the left pulmonary arch) until the period 

 of rapid descent. Then there was a quick approach, so that in a 24-mm. embryo, 

 with the upper part of the sternal bands fused, the two vessels were almost together. 

 They were found in contact in fetuses of less than 40 mm. in length (fig. 17, e). 

 The rapid approximation of the vessels and their final meeting can not be explained 

 by the slowing ingrowth of the wall between them. As in the movement of the 

 innominate and subclavian, there must have been actual progression of the vessels 

 at their origins through the substance of the wall of the parent vessel. 



Bremer (1902, 1909) has made an interesting suggestion as to the nature of 

 the approach of the two pulmonary arteries, based on the observations at later 

 stages in the formation of the adult pulmonary artery and its branches in a number 

 of mammals. He believes that the pulmonary stem undergoes torsion, and that 

 the approach of the two arteries is due to their fusing with it as a result of their 

 proximal ends being wrapped around it. In his second article on the subject, 

 referring to man, rabbit, sheep, and cow, he says: 



"With the growth of thetruncus pulmonalis and its torsion about the bulbus aortse 

 the two pulmonary arches are wound, as it were, around the bulbus and their walls 

 brought into contact are absorbed so that the truncus pulmonalis grows longer at their 

 expense, the point of bifurcation moving continually further from the heart. The left 

 arch being the outside one in this rolling-up process receives the most pull, becomes the 

 straighter and therefore the larger vessel and is shortened more rapidly. As a result 



