86 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



the truncus pulmonalis reaches the left pulmonary artery while the right is still seen 

 arising from the right arch some distance dorsal to this point." 



It can be seen from the described changes in the relative length of the parts 

 of the main pulmonary channel incident upon the formation of the ductus arteri- 

 osus, that the interval marked off by the two pulmonary vessels moves distally 

 on the pulmonary channel. This is as one would expect if there were such a wrap- 

 ping of the pulmonary arteries around it as Bremer describes. As an argument for 

 rotation and wrapping, however, this last circumstance loses much of its force 

 when it is recalled that the segment on the aortic arch between innominate and 

 left common carotid also moves distally on the arch as it grows shorter. Yet there 

 is no reappearance of rotation of the arch nor has such been claimed. The relations 

 of the arteries to the parent stem are not what one would expect were they brought 

 into contact with it and fused as a result of its rotation. In the period preliminary 

 to the rapid descent the primitive pulmonary artery comes off ventro-medially 

 from the stem instead of from the right side, as one might expect from the source 

 of this part of the vessel. The left pulmonary has retained its primitive ventro- 

 medial origin. After the descent the two vessels come off a little more ventrally. 

 This change in the position is of a kind that might have been caused by a slight tor- 

 sion of the stem. To produce wrapping, however, they would have to be carried 

 around much farther to the left. One would also expect their right side to be bent 

 over against the main stem. Instead of this, the two arteries are nearly radial to 

 its cross-section (fig. 17 d). 



We do not know whether the movement of the two pulmonary arteries taken 

 together is due to the shifting in the wall of the pulmonary stem or whether it 

 results from a retardation in the growth of the pulmonary channel distal to them, 

 as was earlier pointed out. The approach of the two vessels is certainly due to a 

 movement of the origin of one or both through the substance of the wall of the 

 main pulmonary stem. It is probable that longitudinal tension of the arteries 

 exerts an important influence in this process, and this, perhaps, may be caused by 

 the elongation of the lung rudiments. It is of interest that the curve of the right 

 pulmonary around the aortic stem is straightened out by the time the two pul- 

 monaries have met, and it may be that the approach of the origin of the two vessels 

 is entirely due to the shifting distally of the origin of this vessel in the course 

 of its straightening. 



By the coming together of the two primitive pulmonary arteries the organ- 

 ization of the pulmonary vessels at the end of prenatal development is closely 

 approached. There is a pulmonary artery giving off a right and a left branch and 

 a ductus arteriosus connecting the latter with the aorta. The pulmonary artery 

 is formed from material derived from the pulmonary trunk and more or less of the 

 left pulmonary arch of the branchial period. The arterial duct owes its origin, in 

 large part at least, to material developed from the distal part of this arch. The 

 two pulmonary arteries are morphologically dissimilar to the extent that the right 

 has a zone produced from the proximal end of the right pulmonary arch, while the 

 left has no corresponding region derived from an arch. 



