AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 81 



have a contrast between the history of the right and left fourth and pulmonary 

 arches, apparently because in each instance the left vessel is now receiving a larger 

 current, and thus can react more vigorously toward its environment than its 

 counterpart on the right. Thus the left fourth moves caudally faster than the 

 right. The left pulmonary, though sharing the pressure of the vagus in its caudal 

 surface, does not undergo involution but is able to move caudally. Apparently 

 it forces the vagus and recurrens before it. Certainly they do elongate the loop 

 which held it so that it can descend to its ultimate position. Most striking of all 

 is the caudal shifting of various vessels, as, for example, the definitive arch. 



Among these various apparent effects of pull exerted by the descending heart 

 and other structures, some, as, for example, the stretching out of vessels into fila- 

 ments at a late stage of involution, are so obviously due to this cause that a dis- 

 cussion is unnecessary. In the early stage of interruption of the arches and the 

 shifting of the arterial branches on their main stems the action of pull is difficult 

 to establish with finality. Experimental evidence or its equivalent (the study of 

 anomalies) is needed. In the sidewise progression of the definitive arch and pul- 

 monary channel there must certainly be factors involved other than the caudal 

 pull at their ends; yet there can be little doubt that in all of the arterial transfor- 

 mations the pull of the heart and shifting of the dorsal aorta are important factors. 



A demonstration of the interplay of longitudinal tension of different confluent 

 vessels has been seen each time a segment of the arch system gave way, and in 

 these instances some of the arteries were showing merely the tension proper to 

 them and entirely independent of a pull due to growth displacements. When one 

 of three converging segments of the system underwent involution and its longitudi- 

 nal tension weakened, the pull of the other two segments overbalanced it, thus 

 stretching it and straightening the angle they formed with one another. This 

 process indicates that under usual conditions the pull of any two such vessels 

 counterbalances the tension exerted by the third. 



A helpful analysis of the movement of the structures of the neck down to and 

 into the thorax has been given by Kingsbury in his study of pharyngeal develop- 

 ment. He describes their displacement to fill the space left vacant by the de- 

 scending heart as a "growth eddy." He points out the complexity of the forces 

 affecting the caudal shifting of the pharyngeal derivatives and expresses his belief 

 that the mesenchyme also moves downward. This is a very appropriate characteri- 

 zation of the movement in its most salient features. It implies, however, a 

 passivity of the structures coming in to occupy the space which probably is not 

 the exclusive condition in any one of them. The arteries seem to act rather 

 vigorously upon their surroundings during their descent. This is indicated by the 

 differences in the relation of the right and left pulmonary arches to the vagus and 

 recurrens nerves which have just been described. The fourth arch also gives 

 evidence of helping to move the structures which earlier barred its way. In the 

 14-mm. embryo a pharyngeal-pouch derivative is found on each side, lying in 

 contact with the caudal surface of the fourth arch, although one of them has moved 



