AOKTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 63 



the arteries which succeed them. The difficulties encountered in tracing their 

 later history are paralleled in the study of the development of other tubular systems, 

 as, for example, the hepatic and pancreatic ducts, or the Wolffian ducts in connection 

 with the urogenital sinus. To follow the material derived from them to its position 

 in the post-branchial vessels, it is necessary to know whether there is, during growth, 

 a fusion or a splitting at the point of bifurcation of vessels, and whether changes in 

 the interval between two lateral branches are due to an alteration in the length of an 

 intervening portion of the main stem or to a more complex shifting of the material 

 by which the branches move bodily along the wall. 



The task of tracing the material of the arch system into the vessels of the post- 

 branchial period is well worth while, not because we expect them to take part as dis- 

 tinguishable units in the adult vessels, but because, on account of the definiteness 

 and multiplicity of the arches and their connections, they are especially good ma- 

 terial for gaining some conception of how rapidly vascular territories in general lose 

 their identity and to what degree their material is intermingled with adjacent 

 regions during development. The history will, at best, be incomplete, since the 

 largest embryo of our series, though its form is far along toward the adult condition, 

 is but 24 mm. in length and must increase about seventyfold before the adult dimen- 

 sions have been reached. 



The breaking up of the arch system of the late branchial period, with its 3 pans 

 of arches, is made possible by its interruption in four regions. This is preceded by a 

 movement of the arches as far caudally as then pharyngeal pouches and other 

 structures allow. The time occupied for each interruption is brief; it can be roughly 

 estimated as a day. The left pulmonary arch is the first to disappear, thus per- 

 mitting the evolution of the pulmonary vessels. The dorsal aorta on each side, 

 between the third and fourth arches, next loses its continuity. This is of especial 

 help in the formation of the definite aortic arch and the innominate and common 

 carotid arteries. Finally, the dorsal aorta, by its interruption close to its caudal end, 

 prepares the way for the remolding of a large part of the right paired aorta, together 

 with the right fourth arch, into the subclavian artery of this side. 



The involution of the right pulmonary arch is confined to the part distal to the 

 origin of the right primitive pulmonary artery (fig. 17, a to d). Models were made 

 of the arch system of 2 embryos in which this region was in a condition of reduced 

 diameter preliminary to its interruption, at the time when evidences of the causes of 

 its degeneration should be most apparent, In fact, indications are not lacking of the 

 presence of mechanical conditions that might cause its involution. The arch seems 

 to be pulled caudally at its ends and held back in its middle portion by the vagus 

 nerve and its recurrent branch. Both ends are bent somewhat caudally and are 

 smaller in diameter than the intermediate part. The upper end comes off the aorta 

 at about the same angle as found at this time in the more cranial segmental arteries, 

 where it is clearly due to the caudal shifting of the aorta relative to the surroundings. 



The existence of a caudal and a transverse pull upon the proximal end is 

 indicated not only by a caudal slope of this segment 'just where it passes down to the 

 origin of the primitive pulmonary artery but also by the rapid withdrawal caudally 



