AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 83 



appearance of passively swinging around to fill the space left vacant by the heart, 

 and this effect is enhanced by its free end swinging downward as well as forward. 

 There is, however, as above indicated, an expansion and a reshaping of its substance, 

 not a participation in a passive eddy movement. It is not improbable that the 

 mechanical influences of the descending heart, which cause the other structures to 

 migrate, have a formative effect upon the growth of the ribs. 



Since the upper ribs move in with other structures to fill the gap left vacant by 

 the heart, the close correlation in time between descent of the heart and closure 

 of the upper thoracic wall becomes understandable. Arterial arrangements which 

 would have tended to crowd the superior thoracic aperture are gradually altered 

 as the heart sinks into the thorax. The movement of the innominate and the 

 left subclavian near to the summit of the aortic arch is of this nature. There is 

 also the bending of the arch so that its dorsoventral diameter is decreased. Most 

 important of all are the changes in the position of the heart. At the beginning of 

 the period, in the 14-mm. embryo, the direction of aortic and pulmonary trunks 

 indicates that the apex of the heart is pointed well forward and that much of its 

 bulk lies ventral to the tip of the ribs. In a 20-mm. embryo the superior thoracic 

 aperture has become closed, and in correlation with this the definitive arch has sunk 

 below the level of the aperture and the heart has swung upon it as a hinge, so that 

 it points more caudally. To have arrived at this position, the apex of the heart 

 must, within a week, have not only moved with the arch at the rate of half a body 

 segment a day but, because of its caudal swing, must have exceeded the arch con- 

 siderably in speed. 



INDIVIDUAL ARTERIES. 

 PULMONARY ARTERY. 



In tracing the development of a blood-vessel its history remains incomplete 

 until one recognizes not only the capillary plexus from which it is derived but also 

 the source of the angioblastic mass giving rise to the capillaries in the event thej' 

 do not arise, directly from an open vessel. In the case of the primitive pulmonary 

 arteries of higher vertebrates, which later evolve into approximately the right and 

 left branches of the definitive pulmonary artery, the manner of origin of the angio- 

 blastic material seems to be well established. Fedorow (1910) and Bremer (1912) 

 trace it in the rabbit and guinea-pig to paired growths from the aortic sac which 

 they believe grow out to form a net from which the pulmonary arch is in turn 

 derived. These authors figure reconstructions of the net. Buell (1922), in the 

 most recent contribution to the development of the pulmonary vessels, which 

 appears in this volume, also traces the angioblastic material in the chick to this 

 source. Huntington (1919) is not in agreement with these observers, as he derives 

 it in the cat from the dorsal aorta. The method used by him in making his prepa- 

 rations is not clear, and the formations figured are too unlike the findings of other 

 writers to constitute satisfactory evidence in support of such a contention. There 

 is also disagreement as to the form and position of the earliest pulmonary vessels 

 themselves. Bremer and Fedorow find that a slender artery first extends caudally 



