AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 53 



bryos, the vessel soon becomes straightened and assumes a median position in the 

 visceral arch. 



Developing pulmonary arches are in our series frequently represented by inde- 

 pendent dorsal and ventral ends (plate 1, figs. 31 and 32). The extension downward 

 from the dorsal aorta lies close behind the caudal pharyngeal complex. 1 Below, 

 a plexus, which earlier can be seen developing caudally from the aortic sac, has given 

 rise to a vessel which has elongated and now extends backward beyond the level 

 of the dorsal sprout, to break up in the pulmonary plexus upon the side of the 

 trachea. The pulmonary arch is completed by an extension of the dorsal sprout 

 which joins the ventral vessel midway in its course, thus dividing it into a proximal 

 portion (now the ventral end of the arch) and a distal portion (the primitive pul- 

 monary artery). Further observations bearing on the development of this arch 

 and the earlier studies on this subject will be referred to in the description of the 

 development of the pulmonary artery. 



The pulmonary arch is more variable as regards the position of its distal end 

 than are the others. As it enters the aorta it may be separated by a distinct interval 

 from the fourth arch (figs. 6, 7) or may be close to it; a common upper end of the 

 two also is frequent. These variations are dependent in part upon changes in the 

 caudal pharyngeal complex, which sometimes lies so near the aorta as to prevent 

 the two arches from close approximation, while at other times it is withdrawn more 

 ventrally. The vagus nerve and its recurrent branch also limit the territory open 

 for occupation by the pulmonary arch on its caudal side, since they pass close behind 

 the caudal pharyngeal complex. 



There have been several studies on the development of the second and suc- 

 ceeding aortic arches by both the reconstruction and the injection methods. The 

 second, third, and fourth arches were found in the rabbit by Bremer (1912) to be 

 preceded by a vascular plexus from the ventral aorta. He described this as po- 

 tentially double for the second arch and multiple for the succeeding arches. 

 Sabin (1917) figures irregular double channels for the second arch in injected 

 chicks. 



In human embryos simple loops (figs. 2, 3), of greater than capillary caliber, 

 not infrequently come off from the aorta at the upper end of the visceral arch 

 before any definite sprout has become established. They may remain for a time 

 as a part of a completed vessel, where they are usually referred to as "island- 

 formations." They were found most frequently in the pulmonary arch, but were 

 also seen in the second, third, and fourth arches. Occasionally they were found in 

 the ventral end of the arch. Lewis (1906), in his discussion of the fifth arch, 

 pointed out that they are of general occurrence in mammals. A survey of the liter- 

 ature on the lower mammals serves to confirm this, and it may be assumed that it is 

 true also of man. It is possible that these loops may be expressions of a tendency 

 toward a double channel in the visceral arches, such as Bremer describes. 



1 This term is applied by Kingsbury to the entire pharyngeal evagination on either side, which lies caudal to the third 

 pharyngeal pouch. 



